diff --git a/core/java/android/content/ContentService.java b/core/java/android/content/ContentService.java
index 974a667..89e504e 100644
--- a/core/java/android/content/ContentService.java
+++ b/core/java/android/content/ContentService.java
@@ -496,6 +496,9 @@
 
             // Look to see if the proper child already exists
             String segment = getUriSegment(uri, index);
+            if (segment == null) {
+                throw new IllegalArgumentException("Invalid Uri (" + uri + ") used for observer");
+            }
             int N = mChildren.size();
             for (int i = 0; i < N; i++) {
                 ObserverNode node = mChildren.get(i);