Index: /CKEditor/trunk/_source/plugins/image/dialogs/image.js
===================================================================
--- /CKEditor/trunk/_source/plugins/image/dialogs/image.js	(revision 6588)
+++ /CKEditor/trunk/_source/plugins/image/dialogs/image.js	(revision 6589)
@@ -638,8 +638,9 @@
 															validate : function()
 															{
-																var aMatch  =  this.getValue().match( regexGetSizeOrEmpty );
-																if ( !aMatch || aMatch[1] == 0 )
+																var aMatch  =  this.getValue().match( regexGetSizeOrEmpty ),
+																	isValid = !!( aMatch && aMatch[ 1 ] != 0 );
+																if ( !isValid )
 																	alert( editor.lang.common.invalidWidth );
-																return !!aMatch;
+																return isValid;
 															},
 															setup : setupDimension,