diff --git a/src/renderers/dom/shared/__tests__/renderSubtreeIntoContainer-test.js b/src/renderers/dom/shared/__tests__/renderSubtreeIntoContainer-test.js
index 43ac5cb5b2b5..cca63bffa77f 100644
--- a/src/renderers/dom/shared/__tests__/renderSubtreeIntoContainer-test.js
+++ b/src/renderers/dom/shared/__tests__/renderSubtreeIntoContainer-test.js
@@ -14,6 +14,7 @@
 var React = require('react');
 var PropTypes = require('prop-types');
 var ReactDOM = require('react-dom');
+var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
 var ReactTestUtils = require('react-dom/test-utils');
 var renderSubtreeIntoContainer = require('react-dom')
   .unstable_renderSubtreeIntoContainer;
@@ -299,4 +300,27 @@ describe('renderSubtreeIntoContainer', () => {
     ReactDOM.render(<Parent value="foo" />, container);
     expect(portal2.textContent).toBe('foo');
   });
+
+  if (ReactDOMFeatureFlags.useFiber) {
+    it('fails gracefully when mixing React 15 and 16', () => {
+      class C extends React.Component {
+        render() {
+          return <div />;
+        }
+      }
+      const c = ReactDOM.render(<C />, document.createElement('div'));
+      // React 15 calls this:
+      // https://github.com/facebook/react/blob/77b71fc3c4/src/renderers/dom/client/ReactMount.js#L478-L479
+      expect(() => {
+        c._reactInternalInstance._processChildContext({});
+      }).toThrow(
+        '_processChildContext is not available in React 16+. This likely ' +
+          'means you have multiple copies of React and are attempting to nest ' +
+          'a React 15 tree inside a React 16 tree using ' +
+          "unstable_renderSubtreeIntoContainer, which isn't supported. Try to " +
+          'make sure you have only one copy of React (and ideally, switch to ' +
+          'ReactDOM.unstable_createPortal).',
+      );
+    });
+  }
 });
diff --git a/src/renderers/shared/fiber/ReactFiberClassComponent.js b/src/renderers/shared/fiber/ReactFiberClassComponent.js
index a6cc6880020a..e726cfd56928 100644
--- a/src/renderers/shared/fiber/ReactFiberClassComponent.js
+++ b/src/renderers/shared/fiber/ReactFiberClassComponent.js
@@ -40,6 +40,7 @@ var getComponentName = require('getComponentName');
 var shallowEqual = require('fbjs/lib/shallowEqual');
 var invariant = require('fbjs/lib/invariant');
 
+const fakeInternalInstance = {};
 const isArray = Array.isArray;
 
 if (__DEV__) {
@@ -54,6 +55,27 @@ if (__DEV__) {
       callback,
     );
   };
+
+  // This is so gross but it's at least non-critical and can be removed if
+  // it causes problems. This is meant to give a nicer error message for
+  // ReactDOM15.unstable_renderSubtreeIntoContainer(reactDOM16Component,
+  // ...)) which otherwise throws a "_processChildContext is not a function"
+  // exception.
+  Object.defineProperty(fakeInternalInstance, '_processChildContext', {
+    enumerable: false,
+    value: function() {
+      invariant(
+        false,
+        '_processChildContext is not available in React 16+. This likely ' +
+          'means you have multiple copies of React and are attempting to nest ' +
+          'a React 15 tree inside a React 16 tree using ' +
+          "unstable_renderSubtreeIntoContainer, which isn't supported. Try " +
+          'to make sure you have only one copy of React (and ideally, switch ' +
+          'to ReactDOM.unstable_createPortal).',
+      );
+    },
+  });
+  Object.freeze(fakeInternalInstance);
 }
 
 module.exports = function(
@@ -283,6 +305,9 @@ module.exports = function(
     workInProgress.stateNode = instance;
     // The instance needs access to the fiber so that it can schedule updates
     ReactInstanceMap.set(instance, workInProgress);
+    if (__DEV__) {
+      instance._reactInternalInstance = fakeInternalInstance;
+    }
   }
 
   function constructClassInstance(workInProgress: Fiber, props: any): any {
diff --git a/src/renderers/shared/shared/ReactInstanceMap.js b/src/renderers/shared/shared/ReactInstanceMap.js
index 495c82c311b6..c26c3c959a16 100644
--- a/src/renderers/shared/shared/ReactInstanceMap.js
+++ b/src/renderers/shared/shared/ReactInstanceMap.js
@@ -26,19 +26,19 @@ var ReactInstanceMap = {
    * supported we can rename it.
    */
   remove: function(key) {
-    key._reactInternalInstance = undefined;
+    key._reactInternalFiber = undefined;
   },
 
   get: function(key) {
-    return key._reactInternalInstance;
+    return key._reactInternalFiber;
   },
 
   has: function(key) {
-    return key._reactInternalInstance !== undefined;
+    return key._reactInternalFiber !== undefined;
   },
 
   set: function(key, value) {
-    key._reactInternalInstance = value;
+    key._reactInternalFiber = value;
   },
 };