diff --git a/mach/z80/libmon/head_em.s b/mach/z80/libmon/head_em.s
index da5dfec3..251bd582 100644
--- a/mach/z80/libmon/head_em.s
+++ b/mach/z80/libmon/head_em.s
@@ -25,6 +25,7 @@
 	ld h,d
 	ld l,e
 	ld bc,begbss
+	xor a			! to clear carry
 	sbc hl,bc
 	ld a,h
 	or l
diff --git a/mach/z80/libsys/head_em.s b/mach/z80/libsys/head_em.s
index da5dfec3..251bd582 100644
--- a/mach/z80/libsys/head_em.s
+++ b/mach/z80/libsys/head_em.s
@@ -25,6 +25,7 @@
 	ld h,d
 	ld l,e
 	ld bc,begbss
+	xor a			! to clear carry
 	sbc hl,bc
 	ld a,h
 	or l