diff --git a/org.adempiere.ui.swing/src/org/compiere/apps/AEnv.java b/org.adempiere.ui.swing/src/org/compiere/apps/AEnv.java
index 35c99fbd79..0f76fc7843 100644
--- a/org.adempiere.ui.swing/src/org/compiere/apps/AEnv.java
+++ b/org.adempiere.ui.swing/src/org/compiere/apps/AEnv.java
@@ -1088,7 +1088,7 @@ public final class AEnv
 	 */
 	public static void removeWindow (int WindowNo)
 	{
-		if (WindowNo < s_windows.size())
+		if (WindowNo>=0 && WindowNo < s_windows.size())
 			s_windows.set(WindowNo, null);
 	}	//	removeWindow