Follow up to #328

Author: leebyron

src/execution/execute.js

@@ -798,11 +798,13 @@ function completeAbstractValue(
   info: GraphQLResolveInfo,
   result: mixed
 ): mixed {
-  let runtimeType: ?GraphQLObjectType;
-  if (returnType.resolveType) {
-    runtimeType = returnType.resolveType(result, info);
-  } else {
-    runtimeType = defaultResolveTypeFn(result, info, returnType);
+  const resolver = returnType.resolveType;
+  const runtimeType = resolver ?
+    resolver(result, info) :
+    defaultResolveTypeFn(result, info, returnType);
+
+  if (!runtimeType) {
+    return null;
   }
 
   if (runtimeType && !returnType.isPossibleType(runtimeType)) {
@@ -813,10 +815,6 @@ function completeAbstractValue(
     );
   }
 
-  if (!runtimeType) {
-    return null;
-  }
-
   return completeObjectValue(
     exeContext,
     runtimeType,