diff --git a/dom/public/idl/geolocation/nsIDOMGeoGeolocation.idl b/dom/public/idl/geolocation/nsIDOMGeoGeolocation.idl
index d30e6971648..278a94e195b 100644
--- a/dom/public/idl/geolocation/nsIDOMGeoGeolocation.idl
+++ b/dom/public/idl/geolocation/nsIDOMGeoGeolocation.idl
@@ -41,7 +41,7 @@ interface nsIDOMGeoPositionOptions;
 interface nsIDOMGeoPositionCallback;
 interface nsIDOMGeoPositionErrorCallback;
 
-[scriptable, function, uuid(CE495440-C8B9-42DE-B67C-60E6928C0F40)]
+[scriptable, function, uuid(0EC70F3F-7E15-45E0-84E9-CDE078CB150A)]
 interface nsIDOMGeoGeolocation : nsISupports
 {
   readonly attribute nsIDOMGeoPosition lastPosition;
@@ -50,9 +50,9 @@ interface nsIDOMGeoGeolocation : nsISupports
                           [optional] in nsIDOMGeoPositionErrorCallback errorCallback,
                           [optional] in nsIDOMGeoPositionOptions options);
 
-  unsigned short watchPosition(in nsIDOMGeoPositionCallback successCallback,
-                               [optional] in nsIDOMGeoPositionErrorCallback errorCallback,
-                               [optional] in nsIDOMGeoPositionOptions options);
+  long watchPosition(in nsIDOMGeoPositionCallback successCallback,
+                     [optional] in nsIDOMGeoPositionErrorCallback errorCallback,
+                     [optional] in nsIDOMGeoPositionOptions options);
 
-  void clearWatch(in unsigned short watchId);
+  void clearWatch(in long watchId);
 };
diff --git a/dom/src/geolocation/nsGeolocation.cpp b/dom/src/geolocation/nsGeolocation.cpp
index 8e8886ff8c6..aac3bf251f8 100644
--- a/dom/src/geolocation/nsGeolocation.cpp
+++ b/dom/src/geolocation/nsGeolocation.cpp
@@ -753,7 +753,7 @@ NS_IMETHODIMP
 nsGeolocation::WatchPosition(nsIDOMGeoPositionCallback *aCallback,
                              nsIDOMGeoPositionErrorCallback *aErrorCallback,
                              nsIDOMGeoPositionOptions *aOptions, 
-                             PRUint16 *_retval NS_OUTPARAM)
+                             PRInt32 *_retval NS_OUTPARAM)
 {
   nsCOMPtr<nsIGeolocationPrompt> prompt = do_GetService(NS_GEOLOCATION_PROMPT_CONTRACTID);
   if (prompt == nsnull)
@@ -778,8 +778,13 @@ nsGeolocation::WatchPosition(nsIDOMGeoPositionCallback *aCallback,
 }
 
 NS_IMETHODIMP
-nsGeolocation::ClearWatch(PRUint16 aWatchId)
+nsGeolocation::ClearWatch(PRInt32 aWatchId)
 {
+  PRUint32 count = mWatchingCallbacks.Length();
+
+  if (aWatchId < 0 || count == 0 || aWatchId > count + 1)
+    return NS_ERROR_FAILURE;
+
   mWatchingCallbacks[aWatchId]->MarkCleared();
   return NS_OK;
 }
diff --git a/dom/tests/mochitest/geolocation/Makefile.in b/dom/tests/mochitest/geolocation/Makefile.in
index 90623bd3ecd..821da080306 100644
--- a/dom/tests/mochitest/geolocation/Makefile.in
+++ b/dom/tests/mochitest/geolocation/Makefile.in
@@ -52,6 +52,7 @@ _TEST_FILES	= \
 		test_cancelCurrent.html \
 		test_cancelWatch.html \
 		test_clearWatch.html \
+		test_clearWatch_invalid.html \
 		test_geoPrompt.html \
 		test_timeoutWatch.html \
 		prompt_common.js       \