--- a/rail_gui.c	Wed Oct 11 21:20:53 2006 +0000
+++ b/rail_gui.c	Wed Oct 11 22:04:46 2006 +0000
@@ -590,6 +590,7 @@
 	Window *w;
 
 	if (_current_player == OWNER_SPECTATOR) return;
+	if (!ValParamRailtype(railtype)) return;
 
 	// don't recreate the window if we're clicking on a button and the window exists.
 	if (button < 0 || !(w = FindWindowById(WC_BUILD_TOOLBAR, 0)) || w->wndproc != BuildRailToolbWndProc) {