diff --git a/src/execution/execute.js b/src/execution/execute.js
index ecf3cff3f2..29e9de0e97 100644
--- a/src/execution/execute.js
+++ b/src/execution/execute.js
@@ -117,6 +117,11 @@ export function execute(
   operationName?: ?string
 ): Promise<ExecutionResult> {
   invariant(schema, 'Must provide schema');
+  invariant(
+    schema instanceof GraphQLSchema,
+    'Schema must be an instance of GraphQLSchema. Also ensure that there are ' +
+    'not multiple versions of GraphQL installed in your node_modules directory.'
+  );
 
   // If a valid context cannot be created due to incorrect arguments,
   // this will throw an error.
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 2b2b9db7aa..2a7f57a75d 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -94,6 +94,10 @@ export function extendSchema(
   schema: GraphQLSchema,
   documentAST: Document
 ): GraphQLSchema {
+  invariant(
+    schema instanceof GraphQLSchema,
+    'Must provide valid GraphQLSchema'
+  );
 
   invariant(
     documentAST && documentAST.kind === DOCUMENT,
diff --git a/src/validation/validate.js b/src/validation/validate.js
index e22ffbf46e..b601e79042 100644
--- a/src/validation/validate.js
+++ b/src/validation/validate.js
@@ -53,6 +53,11 @@ export function validate(
 ): Array<GraphQLError> {
   invariant(schema, 'Must provide schema');
   invariant(ast, 'Must provide document');
+  invariant(
+    schema instanceof GraphQLSchema,
+    'Schema must be an instance of GraphQLSchema. Also ensure that there are ' +
+    'not multiple versions of GraphQL installed in your node_modules directory.'
+  );
   const typeInfo = new TypeInfo(schema);
   return visitUsingRules(schema, typeInfo, ast, rules || specifiedRules);
 }