diff --git a/docshell/base/nsIDocShellContainer.idl b/docshell/base/nsIDocShellContainer.idl
index 07b5a36c6120..210af3ec1e1c 100644
--- a/docshell/base/nsIDocShellContainer.idl
+++ b/docshell/base/nsIDocShellContainer.idl
@@ -37,7 +37,7 @@ interface nsIDocShellContainer : nsISupports
 	The current number of DocShells which are immediate children of the 
 	current DocShell.
 	*/
-	attribute long childCount;
+	readonly attribute long childCount;
 
 	/*
 	Add a new child DocShell.  Adds to the end of the list.