diff --git a/lib/openmsx/share/scripts/boot_env.tcl b/lib/openmsx/share/scripts/boot_env.tcl
index 6ca1bc7..524aeaa 100644
--- a/lib/openmsx/share/scripts/boot_env.tcl
+++ b/lib/openmsx/share/scripts/boot_env.tcl
@@ -42,6 +42,9 @@ proc boot_gui_mode {} {
 	if {$boot_env_renderer_type != 0} {
 		set renderer $boot_env_renderer_type
 		set throttle on
+	} else {
+		puts stderr "error: Requested boot_gui_mode but env.RENDERER_TYPE is missing."
+		exit 1
 	}
 }