diff --git a/src/include/pc80/mc146818rtc.h b/src/include/pc80/mc146818rtc.h
index c74af66..0e15273 100644
--- a/src/include/pc80/mc146818rtc.h
+++ b/src/include/pc80/mc146818rtc.h
@@ -186,10 +186,10 @@
 # include "option_table.h"
 # define CMOS_POST_OFFSET (CMOS_VSTART_cmos_post_offset >> 3)
 #else
-# if defined(CONFIG_CMOS_POST_OFFSET)
+# if defined(CONFIG_CMOS_POST_OFFSET) && CONFIG_CMOS_POST_OFFSET
 #  define CMOS_POST_OFFSET CONFIG_CMOS_POST_OFFSET
 # else
-#  error "Must define CONFIG_CMOS_POST_OFFSET"
+#  error "Must configure CONFIG_CMOS_POST_OFFSET"
 # endif
 #endif