diff --git a/src/execution/index.js b/src/execution/index.js
index cf568c1054..cef50db613 100644
--- a/src/execution/index.js
+++ b/src/execution/index.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  *  Copyright (c) 2015, Facebook, Inc.
  *  All rights reserved.
@@ -10,4 +11,4 @@
 export { execute, defaultFieldResolver, responsePathAsArray } from './execute';
 export { getDirectiveValues } from './values';
 
-export type { ExecutionResult } from './execute';
+export type { ExecutionArgs, ExecutionResult } from './execute';
diff --git a/src/language/lexer.js b/src/language/lexer.js
index 4f8118588c..094ab72bf2 100644
--- a/src/language/lexer.js
+++ b/src/language/lexer.js
@@ -1,4 +1,4 @@
-/* @flow /
+/* @flow */
 /**
  *  Copyright (c) 2015, Facebook, Inc.
  *  All rights reserved.
diff --git a/src/subscription/asyncIteratorReject.js b/src/subscription/asyncIteratorReject.js
index 4aac773fdd..f5c4664a1e 100644
--- a/src/subscription/asyncIteratorReject.js
+++ b/src/subscription/asyncIteratorReject.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  * Copyright (c) 2017, Facebook, Inc.
  * All rights reserved.
@@ -5,8 +6,6 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
- *
- * @flow
  */
 
 import { $$asyncIterator } from 'iterall';
diff --git a/src/subscription/index.js b/src/subscription/index.js
index 2d31f69503..6cddd4790a 100644
--- a/src/subscription/index.js
+++ b/src/subscription/index.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  * Copyright (c) 2017, Facebook, Inc.
  * All rights reserved.
@@ -5,8 +6,6 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
- *
- * @flow
  */
 
 export { subscribe, createSourceEventStream } from './subscribe';
diff --git a/src/subscription/mapAsyncIterator.js b/src/subscription/mapAsyncIterator.js
index c31d74c6cd..238150579c 100644
--- a/src/subscription/mapAsyncIterator.js
+++ b/src/subscription/mapAsyncIterator.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  * Copyright (c) 2017, Facebook, Inc.
  * All rights reserved.
@@ -5,8 +6,6 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
- *
- * @flow
  */
 
 import { $$asyncIterator, getAsyncIterator } from 'iterall';
diff --git a/src/subscription/subscribe.js b/src/subscription/subscribe.js
index f5367e8bd0..103138de12 100644
--- a/src/subscription/subscribe.js
+++ b/src/subscription/subscribe.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  * Copyright (c) 2017, Facebook, Inc.
  * All rights reserved.
@@ -5,8 +6,6 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
- *
- * @flow
  */
 
 import { isAsyncIterable } from 'iterall';
diff --git a/src/validation/index.js b/src/validation/index.js
index c2190f4bbd..e8b5ae1575 100644
--- a/src/validation/index.js
+++ b/src/validation/index.js
@@ -1,3 +1,4 @@
+/* @flow */
 /**
  *  Copyright (c) 2015, Facebook, Inc.
  *  All rights reserved.
diff --git a/src/validation/rules/KnownArgumentNames.js b/src/validation/rules/KnownArgumentNames.js
index 9a2f99e4bd..dcf10e7faf 100644
--- a/src/validation/rules/KnownArgumentNames.js
+++ b/src/validation/rules/KnownArgumentNames.js
@@ -15,17 +15,16 @@ import invariant from '../../jsutils/invariant';
 import suggestionList from '../../jsutils/suggestionList';
 import quotedOrList from '../../jsutils/quotedOrList';
 import * as Kind from '../../language/kinds';
-import type { GraphQLType } from '../../type/definition';
 
 
 export function unknownArgMessage(
   argName: string,
   fieldName: string,
-  type: GraphQLType,
+  typeName: string,
   suggestedArgs: Array<string>
 ): string {
   let message = `Unknown argument "${argName}" on field "${fieldName}" of ` +
-    `type "${String(type)}".`;
+    `type "${typeName}".`;
   if (suggestedArgs.length) {
     message += ` Did you mean ${quotedOrList(suggestedArgs)}?`;
   }
diff --git a/src/validation/rules/PossibleFragmentSpreads.js b/src/validation/rules/PossibleFragmentSpreads.js
index a0d66ac42b..b55df025e4 100644
--- a/src/validation/rules/PossibleFragmentSpreads.js
+++ b/src/validation/rules/PossibleFragmentSpreads.js
@@ -12,6 +12,7 @@ import type { ValidationContext } from '../index';
 import { GraphQLError } from '../../error';
 import { doTypesOverlap } from '../../utilities/typeComparators';
 import { typeFromAST } from '../../utilities/typeFromAST';
+import { isCompositeType } from '../../type/definition';
 import type { GraphQLType } from '../../type/definition';
 
 
@@ -44,8 +45,8 @@ export function PossibleFragmentSpreads(context: ValidationContext): any {
     InlineFragment(node) {
       const fragType = context.getType();
       const parentType = context.getParentType();
-      if (fragType &&
-          parentType &&
+      if (isCompositeType(fragType) &&
+          isCompositeType(parentType) &&
           !doTypesOverlap(context.getSchema(), fragType, parentType)) {
         context.reportError(new GraphQLError(
           typeIncompatibleAnonSpreadMessage(parentType, fragType),