diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index b2ba19a..c21db83 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -1187,7 +1187,7 @@ class Executor
     {
         $serializedResult = $returnType->serialize($result);
 
-        if ($serializedResult === null) {
+        if (Utils::isInvalid($serializedResult)) {
             throw new InvariantViolation(
                 'Expected a value of type "'. Utils::printSafe($returnType) . '" but received: ' . Utils::printSafe($result)
             );
diff --git a/src/Type/Definition/EnumType.php b/src/Type/Definition/EnumType.php
index 035c86f..1f18875 100644
--- a/src/Type/Definition/EnumType.php
+++ b/src/Type/Definition/EnumType.php
@@ -108,7 +108,11 @@ class EnumType extends Type implements InputType, OutputType, LeafType
     public function serialize($value)
     {
         $lookup = $this->getValueLookup();
-        return isset($lookup[$value]) ? $lookup[$value]->name : null;
+        if (isset($lookup[$value])) {
+            return $lookup[$value]->name;
+        }
+
+        return Utils::undefined();
     }
 
     /**
diff --git a/src/Utils/AST.php b/src/Utils/AST.php
index 6fd6a9b..ea38aa4 100644
--- a/src/Utils/AST.php
+++ b/src/Utils/AST.php
@@ -205,15 +205,15 @@ class AST
             return new ObjectValueNode(['fields' => $fieldNodes]);
         }
 
+        Utils::invariant(
+            $type instanceof ScalarType || $type instanceof EnumType,
+            "Must provide Input Type, cannot use: " . Utils::printSafe($type)
+        );
+
         // Since value is an internally represented value, it must be serialized
         // to an externally represented value before converting into an AST.
-        if ($type instanceof LeafType) {
-            $serialized = $type->serialize($value);
-        } else {
-            throw new InvariantViolation("Must provide Input Type, cannot use: " . Utils::printSafe($type));
-        }
-
-        if (null === $serialized) {
+        $serialized = $type->serialize($value);
+        if (null === $serialized || Utils::isInvalid($serialized)) {
             return null;
         }