diff --git a/src/core/ReactCompositeComponent.js b/src/core/ReactCompositeComponent.js
index 5adfae53f4..881d4d9d42 100644
--- a/src/core/ReactCompositeComponent.js
+++ b/src/core/ReactCompositeComponent.js
@@ -26,6 +26,7 @@ var ReactOwner = require('ReactOwner');
 var ReactPerf = require('ReactPerf');
 var ReactPropTransferer = require('ReactPropTransferer');
 var ReactPropTypeLocations = require('ReactPropTypeLocations');
+var ReactPropTypeLocationNames = require('ReactPropTypeLocationNames');
 var ReactUpdates = require('ReactUpdates');
 
 var invariant = require('invariant');
@@ -319,16 +320,46 @@ var RESERVED_SPEC_KEYS = {
     }
   },
   childContextTypes: function(Constructor, childContextTypes) {
+    validateTypeDef(
+      Constructor,
+      childContextTypes,
+      ReactPropTypeLocations.childContext
+    );
     Constructor.childContextTypes = childContextTypes;
   },
   contextTypes: function(Constructor, contextTypes) {
+    validateTypeDef(
+      Constructor,
+      contextTypes,
+      ReactPropTypeLocations.context
+    );
     Constructor.contextTypes = contextTypes;
   },
   propTypes: function(Constructor, propTypes) {
+    validateTypeDef(
+      Constructor,
+      propTypes,
+      ReactPropTypeLocations.prop
+    );
     Constructor.propTypes = propTypes;
   }
 };
 
+function validateTypeDef(Constructor, typeDef, location) {
+  for (var propName in typeDef) {
+    if (typeDef.hasOwnProperty(propName)) {
+      invariant(
+        typeof typeDef[propName] == 'function',
+        '%s: %s type `%s` is invalid; it must be a function, usually from ' +
+        'React.PropTypes.',
+        Constructor.displayName || 'ReactCompositeComponent',
+        ReactPropTypeLocationNames[location],
+        propName
+      );
+    }
+  }
+}
+
 function validateMethodOverride(proto, name) {
   var specPolicy = ReactCompositeComponentInterface[name];
 
@@ -790,9 +821,8 @@ var ReactCompositeComponentMixin = {
   _checkPropTypes: function(propTypes, props, location) {
     var componentName = this.constructor.displayName;
     for (var propName in propTypes) {
-      var checkProp = propTypes[propName];
-      if (checkProp) {
-        checkProp(props, propName, componentName, location);
+      if (propTypes.hasOwnProperty(propName)) {
+        propTypes[propName](props, propName, componentName, location);
       }
     }
   },
diff --git a/src/core/ReactPropTypeLocationNames.js b/src/core/ReactPropTypeLocationNames.js
new file mode 100644
index 0000000000..a62d63a992
--- /dev/null
+++ b/src/core/ReactPropTypeLocationNames.js
@@ -0,0 +1,31 @@
+/**
+ * Copyright 2013 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * @providesModule ReactPropTypeLocationNames
+ */
+
+"use strict";
+
+var ReactPropTypeLocationNames = {};
+
+if (__DEV__) {
+  ReactPropTypeLocationNames = {
+    prop: 'prop',
+    context: 'context',
+    childContext: 'child context'
+  };
+}
+
+module.exports = ReactPropTypeLocationNames;
diff --git a/src/core/ReactPropTypes.js b/src/core/ReactPropTypes.js
index cf243edd86..753d9d73b3 100644
--- a/src/core/ReactPropTypes.js
+++ b/src/core/ReactPropTypes.js
@@ -18,19 +18,11 @@
 
 "use strict";
 
+var ReactPropTypeLocationNames = require('ReactPropTypeLocationNames');
+
 var createObjectFrom = require('createObjectFrom');
 var invariant = require('invariant');
 
-var ReactPropTypeLocationNames = {};
-
-if (__DEV__) {
-  ReactPropTypeLocationNames = {
-    prop: 'prop',
-    context: 'context',
-    childContext: 'child context'
-  };
-}
-
 /**
  * Collection of methods that allow declaration and validation of props that are
  * supplied to React components. Example usage:
diff --git a/src/core/__tests__/ReactCompositeComponent-test.js b/src/core/__tests__/ReactCompositeComponent-test.js
index 188f618b9e..0afef2dbda 100644
--- a/src/core/__tests__/ReactCompositeComponent-test.js
+++ b/src/core/__tests__/ReactCompositeComponent-test.js
@@ -289,6 +289,57 @@ describe('ReactCompositeComponent', function() {
     }).not.toThrow();
   });
 
+  it('should throw on invalid prop types', function() {
+    expect(function() {
+      React.createClass({
+        displayName: 'Component',
+        propTypes: {
+          key: null
+        },
+        render: function() {
+          return <span>{this.props.key}</span>;
+        }
+      });
+    }).toThrow(
+      'Invariant Violation: Component: prop type `key` is invalid; ' +
+      'it must be a function, usually from React.PropTypes.'
+    );
+  });
+
+  it('should throw on invalid context types', function() {
+    expect(function() {
+      React.createClass({
+        displayName: 'Component',
+        contextTypes: {
+          key: null
+        },
+        render: function() {
+          return <span>{this.props.key}</span>;
+        }
+      });
+    }).toThrow(
+      'Invariant Violation: Component: context type `key` is invalid; ' +
+      'it must be a function, usually from React.PropTypes.'
+    );
+  });
+
+  it('should throw on invalid child context types', function() {
+    expect(function() {
+      React.createClass({
+        displayName: 'Component',
+        childContextTypes: {
+          key: null
+        },
+        render: function() {
+          return <span>{this.props.key}</span>;
+        }
+      });
+    }).toThrow(
+      'Invariant Violation: Component: child context type `key` is invalid; ' +
+      'it must be a function, usually from React.PropTypes.'
+    );
+  });
+
   it('should not allow `forceUpdate` on unmounted components', function() {
     var container = document.createElement('div');
     document.documentElement.appendChild(container);