diff --git a/lib/Slic3r/GUI/Tab.pm b/lib/Slic3r/GUI/Tab.pm
index 2633097b5..77477bbbf 100644
--- a/lib/Slic3r/GUI/Tab.pm
+++ b/lib/Slic3r/GUI/Tab.pm
@@ -197,7 +197,7 @@ sub set_dirty {
 
 sub is_dirty {
     my $self = shift;
-    return $self->{dirty};
+    return (defined $self->{dirty});
 }
 
 sub title {