diff --git a/Source/NSKeyValueObserving.m b/Source/NSKeyValueObserving.m
index 47fb3a38c..727b47fd2 100644
--- a/Source/NSKeyValueObserving.m
+++ b/Source/NSKeyValueObserving.m
@@ -1566,7 +1566,9 @@ - (void) removeObserver: (NSObject*)anObserver
 	     forKeyPath: (NSString*)aPath
 		context: (void*)context
 {
-  [self removeObserver: anObserver forKeyPath: aPath];
+  [NSException raise: NSGenericException
+	      format: @"[%@-%@]: This class is not observable",
+    NSStringFromClass([self class]), NSStringFromSelector(_cmd)];
 }
 
 @end