diff --git a/mach/proto/as/comm0.h b/mach/proto/as/comm0.h
index acda1e6a7..56c783a1b 100644
--- a/mach/proto/as/comm0.h
+++ b/mach/proto/as/comm0.h
@@ -89,6 +89,7 @@ _include	<unistd.h>
 #include	<stdio.h>
 #include    <string.h>
 #include	<stdarg.h>
+#include	<errno.h>
 #include	<ctype.h>
 #include	<signal.h>
 #include	<unistd.h>