diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6f5e1fa170c..181ece9df67 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Change log
 
 ### vNEXT
+* Add argument validation in `addMockFunctionsToSchema` for 'schema' property in parameter object [PR #321](https://github.com/apollographql/graphql-tools/pull/321) 
 
 ### v0.11.0
 * Remove dependency on `graphql-subscription` and use an interface for PubSub [PR #295](https://github.com/apollographql/graphql-tools/pull/295)
diff --git a/src/mock.ts b/src/mock.ts
index 4f9a25d6399..27a512e6e77 100644
--- a/src/mock.ts
+++ b/src/mock.ts
@@ -43,9 +43,11 @@ function addMockFunctionsToSchema({ schema, mocks = {}, preserveResolvers = fals
   }
 
   if (!schema) {
-    // XXX should we check that schema is an instance of GraphQLSchema?
     throw new Error('Must provide schema to mock');
   }
+  if (!(schema instanceof GraphQLSchema)) {
+    throw new Error('Value at "schema" must be of type GraphQLSchema');
+  }
   if (!isObject(mocks)) {
     throw new Error('mocks must be of type Object');
   }
diff --git a/src/test/testMocking.ts b/src/test/testMocking.ts
index 21189ad9c49..b8cdefd9401 100644
--- a/src/test/testMocking.ts
+++ b/src/test/testMocking.ts
@@ -90,6 +90,11 @@ describe('Mock', () => {
                   .to.throw('Must provide schema to mock');
   });
 
+  it('throws an error if the property "schema" on the first argument is not of type GraphQLSchema', () => {
+    expect(() => (<any> addMockFunctionsToSchema)({ schema: {}}))
+                  .to.throw('Value at "schema" must be of type GraphQLSchema');
+  });
+
   it('throws an error if second argument is not a Map', () => {
     const jsSchema = buildSchemaFromTypeDefinitions(shorthand);
     expect(() => (<any> addMockFunctionsToSchema)({ schema: jsSchema, mocks: ['a'] }))