diff --git a/src/env.h b/src/env.h
index e0b4322..90b4935 100644
--- a/src/env.h
+++ b/src/env.h
@@ -68,11 +68,6 @@ unused static const char * const _name ## Tests[] = {__VA_ARGS__}; \
 _Static_assert(array_length(_name ## Tests) % 2 == 0, "Array of test strings must have exactly one expected result for each test."); \
 Object _name(Object* params, int length, struct Environment* env)
 
-#define tfn(_name, _symbol, _type, _docs, ...) \
-unused static int (*_name ## TypeChecks[])(Object) = UNPACK _type; \
-static const char * const _name ## Symbol = _symbol; \
-fnn(_name, _docs, __VA_ARGS__)
-
 #define fn(_name, _symbol, _docs, ...) \
 static const char * const _name ## Symbol = _symbol; \
 fnn(_name, _docs, __VA_ARGS__)
diff --git a/src/object.c b/src/object.c
index 79ef2ba..df12bf9 100644
--- a/src/object.c
+++ b/src/object.c
@@ -142,7 +142,7 @@ static const char* errorText[] = {
     "LAMBDA_ARGS_NOT_LIST",
     "DID_NOT_FIND_SYMBOL",
     "BAD_TYPE",
-    "BAD_PARAMS_ON",
+    "BAD_PARAMS",
     "BAD_NUMBER",
     "UNSUPPORTED_NUMBER_TYPE",
     "NOT_ENOUGH_ARGUMENTS",
@@ -332,14 +332,12 @@ char* stringObj(const Object* obj, size_t* length)
 #if defined(DEBUG) || defined(STANDALONE)
 
 #define SIMPLE_TYPE(_type) case _type:\
-    printf(#_type);\
-    return
+    return #_type;
 
-void printType(const Object* obj)
+const char* getTypeName(const Object* obj)
 {
     if (!obj) {
-        printf("NULL OBJECT");
-        return;
+        return "NULL_OBJECT";
     }
     switch (obj->type) {
         SIMPLE_TYPE(TYPE_NUMBER);
@@ -353,14 +351,10 @@ void printType(const Object* obj)
         SIMPLE_TYPE(TYPE_PROMISE);
         SIMPLE_TYPE(TYPE_OTHER);
         SIMPLE_TYPE(TYPE_ERROR);
-        case TYPE_LAMBDA:
-            printf("TYPE_LAMBDA Params:\n");
-            return;
+        SIMPLE_TYPE(TYPE_LAMBDA);
     }
 
-    if (!isValidType(*obj)) {
-        printf("UNKNOWN TYPE (%d)", obj->type);
-    }
+    return "UNKNOWN_TYPE";
 }
 
 void _printObj(const Object* obj, int newline)
diff --git a/src/object.h b/src/object.h
index a0a78e2..83e615d 100644
--- a/src/object.h
+++ b/src/object.h
@@ -46,7 +46,7 @@ enum errorCode {
     LAMBDA_ARGS_NOT_LIST,
     DID_NOT_FIND_SYMBOL,
     BAD_TYPE,
-    BAD_PARAMS_ON,
+    BAD_PARAMS,
     BAD_NUMBER,
     UNSUPPORTED_NUMBER_TYPE,
     NOT_ENOUGH_ARGUMENTS,
@@ -147,7 +147,7 @@ struct string {
  */
 char* stringObj(const Object* obj, size_t* length);
 
-void printType(const Object* obj);
+const char* getTypeName(const Object* obj);
 
 void printObj(const Object* obj);
 
diff --git a/src/pebblisp.c b/src/pebblisp.c
index 6fb5006..2676a40 100644
--- a/src/pebblisp.c
+++ b/src/pebblisp.c
@@ -520,15 +520,18 @@ Object parseEval(const char* input, struct Environment* env)
 }
 
 Object typeCheck(const char* funcName, Object* params, int length,
-                 int (* typeChecks[])(Object), int typeLength, int* failed)
+                 struct TypeCheck typeChecks[], int typeLength, int* failed)
 {
     *failed = 1;
     if ((typeLength - 1) > length) {
         return errorWithContext(NOT_ENOUGH_ARGUMENTS, funcName);
     }
+
     for (int i = 0; i < typeLength - 1; i++) {
-        if (typeChecks[i] && !typeChecks[i](params[i])) { // TODO: Use pl func name instead of C function name.
-            return /*errorObject(BAD_TYPE); */ errorWithContextLineNo(BAD_PARAMS_ON, funcName, 0, NULL);
+        if (typeChecks[i].checkFunc && !typeChecks[i].checkFunc(params[i])) { // TODO: Use pl func name instead of C function name.
+            char context[128];
+            sprintf(context, "When calling %s, expected %s, but received %s", funcName, typeChecks[i].name, getTypeName(&params[i]));
+            return errorWithContextLineNo(BAD_PARAMS, context, 0, NULL);
         }
     }
     *failed = 0;
diff --git a/src/pebblisp.h b/src/pebblisp.h
index af3d3ac..f4ff740 100644
--- a/src/pebblisp.h
+++ b/src/pebblisp.h
@@ -19,8 +19,17 @@ unused static const char * const _name ## Tests[] = {__VA_ARGS__}; \
 _Static_assert(array_length(_name ## Tests) % 2 == 0, "Array of test strings must have exactly one expected result for each test."); \
 Object _name(Object* params, int length, struct Environment* env)
 
+struct TypeCheck {
+    int (*checkFunc)(Object);
+    const char* name;
+};
+
+#define expect(_checker) {.checkFunc = (_checker), .name = #_checker}
+#define returns(_checker) {.checkFunc = (_checker), .name = #_checker}
+#define anyType {.checkFunc = NULL, .name = "AnyType"}
+
 #define tfn(_name, _symbol, _type, _docs, ...) \
-unused static int (*_name ## TypeChecks[])(Object) = UNPACK _type; \
+unused static struct TypeCheck _name ## TypeChecks[] = UNPACK _type; \
 static const char * const _name ## Symbol = _symbol; \
 fnn(_name, _docs, __VA_ARGS__)
 
@@ -76,7 +85,7 @@ Object listEvalLambda(Object* lambda, const Object* remaining, int evalLength,
 Object simpleFuncEval(Object func, Object arg1, Object arg2, struct Environment* env);
 
 Object typeCheck(const char* funcName, Object* params, int length,
-                 int (* typeChecks[])(Object), int typeLength, int* failed);
+                 struct TypeCheck typeChecks[], int typeLength, int* failed);
 
 #ifndef STANDALONE
 #define DISABLE_TYPE_CHECKS
@@ -112,7 +121,7 @@ fn(def, "def",
 );
 
 tfn(structAccess, "poss",
-    ({ isStruct, isStringy, NULL }),
+    ({ expect(isStruct), expect(isStringy), anyType }),
     "Get the value of a struct's field",
     "(struct Post (title body))\n   "
     "(def p (Post \"TI\" \"BO\"))\n   "
diff --git a/src/plfunc.c b/src/plfunc.c
index d14e3e0..602c1bc 100644
--- a/src/plfunc.c
+++ b/src/plfunc.c
@@ -251,7 +251,7 @@ Object substring(Object* params, int length, unused struct Environment* env)
 
     int len = end.number - start.number;
     if (len < 0 || start.number >= strlen(string.string)) {
-        return errorWithContext(BAD_PARAMS_ON, string.string);
+        return errorWithContext(BAD_PARAMS, string.string);
     }
     Object substr = withLen(len, TYPE_STRING);
     snprintf(substr.string, len + 1, "%s", string.string + start.number);
diff --git a/src/plfunc.h b/src/plfunc.h
index 56c7feb..cddb1d2 100644
--- a/src/plfunc.h
+++ b/src/plfunc.h
@@ -68,40 +68,40 @@ fn(or, "|",
 );
 
 tfn(catObjects, "cat",
-    ({ NULL, isStringy }),
+    ({ anyType, returns(isStringy) }),
     "Concatenate string representations of the given objects.",
     "(cat \"Stuff: \" (1 2 3))", "Stuff: ( 1 2 3 )",
 );
 
 tfn(filter, "fil",
-    ({ isFuncy, isListy, isListy }),
+    ({ expect(isFuncy), expect(isListy), returns(isListy) }),
     "Filter a list based on the given condition.",
     "(fil (fn (a) (< 50 a)) (25 60 100))", "( 60 100 )",
     "(fil (fn (a) (< 0 (len a))) ( () (1) (1 2) () ))", "( ( 1 ) ( 1 2 ) )",
 );
 
 tfn(append, "ap",
-    ({ isListy, NULL, isListy }),
+    ({ expect(isListy), anyType, returns(isListy) }),
     "Append the given element. Creates a new list.",
     "(ap (1 2) 3)", "( 1 2 3 )",
 );
 
 tfn(prepend, "pre",
-    ({ isListy, NULL, isListy }),
+    ({ expect(isListy), anyType, returns(isListy) }),
     "Prepend the given element. Creates a new list",
     "(pre (2 3) 1)", "( 1 2 3 )",
 );
 
 tfn(len, "len",
-    ({ isListy, isNumber }),
+    ({ expect(isListy), returns(isNumber) }),
     "Returns the length of the given list, or an error if the expression is not a list.",
     "(len (2 3))", "2",
     "(len ())", "0",
-    "(len \"string\")", "BAD_PARAMS_ON: len",
+    "(len \"string\")", "BAD_PARAMS;",
 );
 
 tfn(reduce, "reduce",
-    ({ NULL, isFuncy, NULL, NULL }),
+    ({ anyType, expect(isFuncy), anyType, anyType }),
     "Performs a simple reduction. Does not currently work with lambdas.\n"
     "Takes three arguments:\n"
     " - Values\n"
@@ -112,30 +112,30 @@ tfn(reduce, "reduce",
 );
 
 tfn(at, "at",
-    ({ isNumber, isListy, NULL }),
+    ({ expect(isNumber), expect(isListy), anyType }),
     "Get item at the given index in the given list.",
     "(at 1 (1 2 3))", "2",
     "(at 99 (1 2 3))", "INDEX_PAST_END",
-    "(at 99 \"string\")", "BAD_PARAMS_ON: at",
+    "(at 99 \"string\")", "BAD_PARAMS;",
 );
 
 tfn(rest, "rest",
-    ({ isListy, isListy }),
+    ({ expect(isListy), returns(isListy) }),
     "Get the tail of a list. All but the first element.",
     "(rest (1 2 3))", "( 2 3 )",
     "(rest ())", "( )",
-    "(rest \"string\")", "BAD_PARAMS_ON: rest",
+    "(rest \"string\")", "BAD_PARAMS;",
 );
 
 tfn(reverse, "rev",
-    ({ isListy, isListy }),
+    ({ expect(isListy), returns(isListy) }),
     "Reverse a list.",
     "(rev (1 2 3))", "( 3 2 1 )",
-    "(rev \"string\")", "BAD_PARAMS_ON: reverse",
+    "(rev \"string\")", "BAD_PARAMS;",
 );
 
 tfn(isNum, "isnum",
-    ({ NULL, isBool }),
+    ({ anyType, returns(isBool) }),
     "Returns `T` only if the argument evaluates to a number.",
     "(isnum 1)", "T",
     "(isnum (+ 5 5))", "T",
@@ -144,7 +144,7 @@ tfn(isNum, "isnum",
 );
 
 tfn(isList, "islist",
-    ({ NULL, isBool }),
+    ({ anyType, returns(isBool) }),
     "Returns `T` only if the argument is a list.",
     "(islist (1 2 3))", "T",
     "(islist ())", "T",
@@ -152,7 +152,7 @@ tfn(isList, "islist",
 );
 
 tfn(isString, "isstr",
-    ({ NULL, isBool }),
+    ({ anyType, returns(isBool) }),
     "Returns `T` only if the argument is a string.",
     "(isstr \"Heyo\")", "T",
     "(isstr \"\")", "T",
@@ -161,21 +161,21 @@ tfn(isString, "isstr",
 );
 
 tfn(isErr, "iserr",
-    ({ NULL, isBool }),
+    ({ anyType, returns(isBool) }),
     "Check if the argument is an error.",
     "(iserr (at 10 ()))", "T",
     "(iserr 5)", "F",
 );
 
 tfn(charAt, "chat",
-    ({ isStringy, isNumber, isStringy }),
+    ({ expect(isStringy), expect(isNumber), returns(isStringy) }),
     "Get the char in the given string at the given index.",
     "(chat \"Hello\" 1)", "e",
     "(chat \"Hello\" 10)", "",
 );
 
 tfn(charVal, "char",
-    ({ isStringy, isNumber }),
+    ({ expect(isStringy), returns(isNumber) }),
     "Get the ascii integer representaton of the given character.",
     "(char \"h\")", "104",
     "(char \"hello\")", "104",
@@ -183,21 +183,21 @@ tfn(charVal, "char",
 );
 
 tfn(slen, "slen",
-    ({ isStringy, isNumber }),
+    ({ expect(isStringy), returns(isNumber) }),
     "Returns the length of the given string",
     "(slen \"string\")", "6",
     "(slen \"\")", "0",
 );
 
 tfn(chars, "chars",
-    ({ isStringy, isListy }),
+    ({ expect(isStringy), returns(isListy) }),
     "Get a list of all chars in the given string",
     "(chars \"hello\")", "( h e l l o )",
     "(chars \"\")", "( )",
 );
 
 tfn(matches, "matches",
-    ({ isStringy, isStringy, isBool }),
+    ({ expect(isStringy), expect(isStringy), returns(isBool) }),
     "Check that a string matches a basic wildcard pattern\n"
     "Note: Currently there is no way to match a literal asterisk",
     "(matches \"Hiya\" \"Hiya\")", "T",
@@ -214,11 +214,11 @@ tfn(matches, "matches",
 );
 
 tfn(substring, "substr",
-    ({ isNumber, isNumber, isStringy, isStringy }),
+    ({ expect(isNumber), expect(isNumber), expect(isStringy), returns(isStringy) }),
     "Get a substring from the given string.",
     "(substr 1 3 \"Hello\")", "el",
-    "(substr 99 3 \"Hello\")", "BAD_PARAMS_ON: Hello;",
-    "(substr 98 99 \"Hello\")", "BAD_PARAMS_ON: Hello;",
+    "(substr 99 3 \"Hello\")", "BAD_PARAMS;",
+    "(substr 98 99 \"Hello\")", "BAD_PARAMS;",
     "(substr 0 1 \"\")", "",
     "(substr 0 99 \"Hey\")", "Hey",
     "(substr 1 99 \"Heyyyy\")", "eyyyy",
@@ -232,7 +232,7 @@ fn(parseEvalO, "eval",
 );
 
 tfn(getTime, "time",
-    ({ isStruct }),
+    ({ returns(isStruct) }),
     "Get a struct of the current time with fields (minute hour sec)."
 );
 
@@ -241,7 +241,7 @@ tfn(getTime, "time",
 fn(print, "prn", "Prints the string representation of the given object to stdout.");
 
 tfn(numToChar, "ch",
-    ({ isNumber, isStringy }),
+    ({ expect(isNumber), returns(isStringy) }),
     "Gets a string containing the ascii character for the given number value.",
     "(ch 107)", "k",
     "(ch 0x21)", "!",
@@ -255,14 +255,14 @@ fn(printEnvO, "penv",
 );
 
 tfn(systemCall, "sys",
-    ({ isStringy, isNumber }),
+    ({ expect(isStringy), returns(isNumber) }),
     "Opens a shell and runs the given command, returning 0 if successful.\n"
     "If the argument is not a string, returns 255.\n",
     "(sys \"echo yee > /dev/null\")", "0",
 );
 
 tfn(loadFile, "loadfile",
-    ({ isStringy, NULL }),
+    ({ expect(isStringy), anyType }),
     "Loads and parses the given file.\n"
     "Returns 0 if the file was loaded and parsed successfully. Otherwise 1.\n"
     "(loadfile \"printdate.pl\")\n"
@@ -271,12 +271,12 @@ tfn(loadFile, "loadfile",
 );
 
 tfn(cd, "cd",
-    ({ isStringy, NULL }),
+    ({ expect(isStringy), anyType }),
     "Change the current directory."
 );
 
 tfn(cwd, "cwd",
-    ({ isStringy }),
+    ({ returns(isStringy) }),
     "Get the current directory."
 );
 
@@ -290,12 +290,12 @@ fn(takeInput, "inp",
 );
 
 tfn(readFileToObject, "rf",
-    ({ isStringy, isStringy }),
+    ({ expect(isStringy), returns(isStringy) }),
     "Read a file into a string object."
 );
 
 tfn(getEnvVar, "env",
-    ({ isStringy, isStringy }),
+    ({ expect(isStringy), returns(isStringy) }),
     "Get a variable from the current environment\n"
     "(env HOME) => /home/sagevaillancourt"
 );
diff --git a/src/tests.sh b/src/tests.sh
index f72d655..e65e84a 100755
--- a/src/tests.sh
+++ b/src/tests.sh
@@ -65,13 +65,13 @@ check() {
     local exit_code=0
     if $VALGRIND; then
         echo -ne "\n   $1"
-        output="$($VALCOM ./pl "(loadfile \"examples/lib.pbl\") $2")"
+        output="$($VALCOM ./pl --ignore-config "(loadfile \"examples/lib.pbl\") $2")"
         exit_code=$?
         if [[ "$exit_code" == "0" ]]; then
             echo -ne "\r "
         fi
     else
-        output="$(./pl "(loadfile \"examples/lib.pbl\") $2")"
+        output="$(./pl --ignore-config "(loadfile \"examples/lib.pbl\") $2")"
     fi
 
 
@@ -217,7 +217,7 @@ check "FuncReturningAFunc" "(def plusser (fn (outer) (fn (inner) (+ outer inner)
     (plusFive 10)" "15"
 
 title "ShouldError"
-check "LenOfNotList" "(len 5)" regex "BAD_PARAMS_ON.*"
+check "LenOfNotList" "(len 5)" regex "BAD_PARAMS.*"
 check "NoMapList" "(map sq)" "NULL_MAP_ARGS"
 check "BadNumber" "(5df)" regex "BAD_NUMBER.*"
 check "BadHex" "(0x0zf)" regex "BAD_NUMBER.*"
diff --git a/src/threads.c b/src/threads.c
index 815e623..d89dc5f 100644
--- a/src/threads.c
+++ b/src/threads.c
@@ -25,6 +25,11 @@ int isDone(struct Promise* promise)
     return promise->done;
 }
 
+int isPromise(Object src)
+{
+    return src.type == TYPE_PROMISE;
+}
+
 void cleanPromise(struct Promise* promise)
 {
     promise->refs -= 1;
@@ -36,6 +41,7 @@ void cleanPromise(struct Promise* promise)
 
 Object await(Object* params, int length, struct Environment* env)
 {
+    checkTypes(await)
     struct Promise* promise = params[0].promise;
     if (!promise->done) { // TODO: Does `done` need a mutex or other lock?
         pthread_join(promise->thread, NULL);
@@ -67,7 +73,7 @@ Object async(Object* params, int length, struct Environment* env)
         .refs = 2,
         .done = 0,
         .env = env,
-        .object = params[0], // TODO: Clone?
+        .object = cloneObject(params[0]),
     };
     env->refs += 1;
 
diff --git a/src/threads.h b/src/threads.h
index 22de236..8e198a7 100644
--- a/src/threads.h
+++ b/src/threads.h
@@ -11,15 +11,18 @@ void cleanPromise(struct Promise* promise);
 
 int isDone(struct Promise* promise);
 
+int isPromise(Object src);
+
 fn(async, "async",
    "Run the given lambda on a separate thread, returning a promise.",
    "(def sleepy (fn () ((sys \"sleep 0.01\") \"Hiya\"))) (def x (async sleepy)) x", "<PENDING>",
    "(def sleepy (fn () ((sys \"sleep 0.01\") \"Hiya\"))) (def x (async sleepy)) (await x)", "Hiya",
 );
 
-fn(await, "await",
-   "Waits for a promise to resolve before proceeding.",
-   "(def sleepy (fn () ((sys \"sleep 0.01\") \"Hiya\"))) (def x (async sleepy)) (await x)", "Hiya",
+tfn(await, "await",
+    ({ expect(isPromise), anyType }),
+    "Waits for a promise to resolve before proceeding.",
+    "(def sleepy (fn () ((sys \"sleep 0.01\") \"Hiya\"))) (def x (async sleepy)) (await x)", "Hiya",
 );
 
 #endif // PEBBLISP_THREADS_H