diff --git a/mach/z80/as/mach0.c b/mach/z80/as/mach0.c
index 0b334c5eb..25c518fe4 100644
--- a/mach/z80/as/mach0.c
+++ b/mach/z80/as/mach0.c
@@ -6,3 +6,6 @@
 #define	THREE_PASS		/* distinguish short and long branches */
 #define	LISTING			/* enable listing facilities */
 #define RELOCATION
+
+#undef ISALNUM
+#define ISALNUM(c)	(isalnum(c) || c == '_' || c == '.')