Index: parse/sexp.cpp
===================================================================
--- parse/sexp.cpp	(revision 5529)
+++ parse/sexp.cpp	(working copy)
@@ -13699,7 +13699,7 @@
 		Warning(LOCATION, "Support ship class '%s' not found.\n", CTEXT(n));
 		return;
 	}
-	if (!(Ship_info[temp_val].flags & SIF_SUPPORT))
+	if ((temp_val >= 0) && !(Ship_info[temp_val].flags & SIF_SUPPORT))
 	{
 		Warning(LOCATION, "Ship %s is not a support ship!", Ship_info[temp_val].name);
 		return;