diff --git a/src/group_gui.cpp b/src/group_gui.cpp
--- a/src/group_gui.cpp
+++ b/src/group_gui.cpp
@@ -780,6 +780,8 @@ static const WindowDesc _group_desc = {
 
 void ShowPlayerGroup(PlayerID player, VehicleType vehicle_type)
 {
+	if (!IsValidPlayer(player)) return;
+
 	WindowClass wc;
 
 	switch (vehicle_type) {