diff --git a/Source/HBIOS/usrrom.asm b/Source/HBIOS/usrrom.asm
index 1978ce5c..680fbc79 100644
--- a/Source/HBIOS/usrrom.asm
+++ b/Source/HBIOS/usrrom.asm
@@ -190,6 +190,11 @@ MESSAGE:
 ; ERROR IF THE SIZE EXCEEDS THE SPACE ALLOCATED.
 ;
 SLACK	.EQU	(USR_END - $)
+;
+#IF (SLACK < 0)
+	.ECHO	"*** USRAPP IS TOO BIG!!!\n"
+	!!!	; FORCE AN ASSEMBLY ERROR
+#ENDIF
 ;
 	.FILL	SLACK,$00
 	.ECHO	"User ROM space remaining: "