diff --git a/apps/zasm/instr.asm b/apps/zasm/instr.asm
index af27bdf..52dff9c 100644
--- a/apps/zasm/instr.asm
+++ b/apps/zasm/instr.asm
@@ -180,6 +180,7 @@ parseArg:
 	; note: the "-" part isn't supported yet.
 	inc	hl	; (HL) now points to X or Y
 	ld	a, (hl)
+	call	upcase
 	inc	hl	; advance HL to the number part
 	inc	hl	; this is the number
 	cp	'Y'
diff --git a/tools/emul/zasm/zasm.bin b/tools/emul/zasm/zasm.bin
index 7c5a6fa..98c83b5 100644
Binary files a/tools/emul/zasm/zasm.bin and b/tools/emul/zasm/zasm.bin differ
diff --git a/tools/tests/zasm/test6.asm b/tools/tests/zasm/test6.asm
new file mode 100644
index 0000000..d91189e
--- /dev/null
+++ b/tools/tests/zasm/test6.asm
@@ -0,0 +1,6 @@
+; There was a very nasty bug (cost me a couple of hours of mis-debugging) where
+; ld r, (iy+d) would use the 0xdd byte code instead of the correct 0xfd one.
+; genallinstrs didn't catch it because it outputs uppercase. Oh sweet mother,
+; how much time did I lose over this...
+
+ld h, (iy+1)
diff --git a/tools/tests/zasm/test6.asm.expected b/tools/tests/zasm/test6.asm.expected
new file mode 100644
index 0000000..ca54089
--- /dev/null
+++ b/tools/tests/zasm/test6.asm.expected
@@ -0,0 +1 @@
+ýf

\ No newline at end of file