diff -r f0300bf50643 -r 1448c1051d40 src/group_cmd.cpp
--- a/src/group_cmd.cpp	Wed May 23 13:52:10 2007 +0000
+++ b/src/group_cmd.cpp	Wed May 23 14:02:10 2007 +0000
@@ -205,8 +205,10 @@
 
 	if (!IsValidVehicleID(p2) || (!IsValidGroupID(new_g) && !IsDefaultGroupID(new_g))) return CMD_ERROR;
 
-	Group *g = GetGroup(new_g);
-	if (g->owner != _current_player) return CMD_ERROR;
+	if (IsValidGroupID(new_g)) {
+		Group *g = GetGroup(new_g);
+		if (g->owner != _current_player) return CMD_ERROR;
+	}
 
 	Vehicle *v = GetVehicle(p2);
 	if (v->owner != _current_player || (v->type == VEH_TRAIN && !IsFrontEngine(v))) return CMD_ERROR;