diff --git a/src/vm/wren_primitive.c b/src/vm/wren_primitive.c
index 3671e22f..6ead2867 100644
--- a/src/vm/wren_primitive.c
+++ b/src/vm/wren_primitive.c
@@ -45,7 +45,7 @@ bool validateInt(WrenVM* vm, Value arg, const char* argName)
   return validateIntValue(vm, AS_NUM(arg), argName);
 }
 
-bool validateKey(WrenVM* vm, Value arg)
+inline bool validateKeyType(WrenVM* vm, Value arg)
 {
   if (IS_BOOL(arg) || IS_CLASS(arg) || IS_NULL(arg) ||
       IS_NUM(arg) || IS_RANGE(arg) || IS_STRING(arg))
@@ -53,6 +53,13 @@ bool validateKey(WrenVM* vm, Value arg)
     return true;
   }
 
+  return false;
+}
+
+bool validateKey(WrenVM* vm, Value arg)
+{
+  if (validateKeyType(vm, arg)) return true;
+
   RETURN_ERROR("Key must be a value type.");
 }
 
diff --git a/src/vm/wren_primitive.h b/src/vm/wren_primitive.h
index eb55fe84..4b7c9346 100644
--- a/src/vm/wren_primitive.h
+++ b/src/vm/wren_primitive.h
@@ -79,9 +79,14 @@ bool validateIntValue(WrenVM* vm, double value, const char* argName);
 // reports an error and returns false.
 bool validateInt(WrenVM* vm, Value arg, const char* argName);
 
+// Validates that [arg] is a valid object for use as a map key. Returns true if
+// it is and returns false otherwise. Use validateKey usually, for a runtime error.
+// This separation exists to aid the API in surfacing errors to the developer as well.
+bool validateKeyType(WrenVM * vm, Value arg);
+
 // Validates that [arg] is a valid object for use as a map key. Returns true if
 // it is. If not, reports an error and returns false.
-bool validateKey(WrenVM* vm, Value arg);
+bool validateKey(WrenVM * vm, Value arg);
 
 // Validates that the argument at [argIndex] is an integer within `[0, count)`.
 // Also allows negative indices which map backwards from the end. Returns the
diff --git a/src/vm/wren_vm.c b/src/vm/wren_vm.c
index c53f2090..c1f7ce8f 100644
--- a/src/vm/wren_vm.c
+++ b/src/vm/wren_vm.c
@@ -1826,6 +1826,7 @@ bool wrenGetMapContainsKey(WrenVM* vm, int mapSlot, int keySlot)
   ASSERT(IS_MAP(vm->apiStack[mapSlot]), "Slot must hold a map.");
 
   Value key = vm->apiStack[keySlot];
+  ASSERT(validateKeyType(vm, key), "Key must be a value type");
   if (!validateKey(vm, key)) return false;
 
   ObjMap* map = AS_MAP(vm->apiStack[mapSlot]);
@@ -1858,6 +1859,8 @@ void wrenSetMapValue(WrenVM* vm, int mapSlot, int keySlot, int valueSlot)
   ASSERT(IS_MAP(vm->apiStack[mapSlot]), "Must insert into a map.");
   
   Value key = vm->apiStack[keySlot];
+  ASSERT(validateKeyType(vm, key), "Key must be a value type");
+
   if (!validateKey(vm, key)) {
     return;
   }