Index: jquery/src/ajax.js
===================================================================
--- jquery/src/ajax.js	(revision 5830)
+++ jquery/src/ajax.js	(working copy)
@@ -283,11 +283,16 @@
 
 		// Open the socket
 		// Passing null username, generates a login popup on Opera (#2865)
-		if( s.username )
-			xhr.open(type, s.url, s.async, s.username, s.password);
-		else
-			xhr.open(type, s.url, s.async);
 
+		try {
+			if( s.username )
+				xhr.open(type, s.url, s.async, s.username, s.password);
+			else
+				xhr.open(type, s.url, s.async);
+		} catch (e) {
+			return jQuery.handleError(s, xhr, "error", e);
+		}
+
 		// Need an extra try/catch for cross domain requests in Firefox 3
 		try {
 			// Set the correct header, if data is being sent