Warn if the included mixin is undefined

src/isomorphic/classic/class/ReactClass.js

@@ -437,6 +437,21 @@ function validateMethodOverride(isAlreadyDefined, name) {
  */
 function mixSpecIntoComponent(Constructor, spec) {
   if (!spec) {
+    if (__DEV__) {
+      var typeofSpec = typeof spec;
+      var isMixinValid = typeofSpec === 'object' && spec !== null;
+
+      warning(
+        isMixinValid,
+        '%s: You\'re attempting to include a mixin that is either null ' +
+        'or not an object. Check the mixins included by the component, ' +
+        'as well as any mixins they include themselves. ' +
+        'Expected object but got %s.',
+        Constructor.displayName || 'ReactClass',
+        spec === null ? null : typeofSpec
+      );
+    }
+
     return;
   }
 

src/isomorphic/classic/class/__tests__/ReactClassMixin-test.js

@@ -310,6 +310,46 @@ describe('ReactClass-mixin', function() {
     );
   });
 
+  it('should warn if the mixin is undefined', function() {
+    spyOn(console, 'error');
+
+    React.createClass({
+      mixins: [undefined],
+
+      render: function() {
+        return <span />;
+      },
+    });
+
+    expect(console.error.argsForCall.length).toBe(1);
+    expect(console.error.argsForCall[0][0]).toBe(
+      'Warning: ReactClass: You\'re attempting to include a mixin that is ' +
+      'either null or not an object. Check the mixins included by the ' +
+      'component, as well as any mixins they include themselves. ' +
+      'Expected object but got undefined.'
+    );
+  });
+
+  it('should warn if the mixin is null', function() {
+    spyOn(console, 'error');
+
+    React.createClass({
+      mixins: [null],
+
+      render: function() {
+        return <span />;
+      },
+    });
+
+    expect(console.error.argsForCall.length).toBe(1);
+    expect(console.error.argsForCall[0][0]).toBe(
+      'Warning: ReactClass: You\'re attempting to include a mixin that is ' +
+      'either null or not an object. Check the mixins included by the ' +
+      'component, as well as any mixins they include themselves. ' +
+      'Expected object but got null.'
+    );
+  });
+
   it('should throw if the mixin is a React component', function() {
     expect(function() {
       React.createClass({