diff --git a/pcbnew/dialogs/panel_setup_rules.cpp b/pcbnew/dialogs/panel_setup_rules.cpp
index 40b7446c68..7640419b2b 100644
--- a/pcbnew/dialogs/panel_setup_rules.cpp
+++ b/pcbnew/dialogs/panel_setup_rules.cpp
@@ -114,7 +114,9 @@ void PANEL_SETUP_RULES::onScintillaCharAdded( wxStyledTextEvent &aEvent )
         }
         else if( c == ')' )
         {
-            sexprs.pop();
+            if( !sexprs.empty() )
+                sexprs.pop();
+
             context = NONE;
         }
         else if( c == ' ' )