diff --git a/include/libbb.h b/include/libbb.h
index fb7296f..ad0d59d 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1579,9 +1579,10 @@
 #undef isupper
 #undef isxdigit
 
-/* This one is more efficient - we save ~400 bytes */
+/* This one is more efficient - we save ~500 bytes.
+ * BTW, x86 likes (unsigned char) cast more than (unsigned). */
 #undef isdigit
-#define isdigit(a) ((unsigned)((a) - '0') <= 9)
+#define isdigit(a) ((unsigned char)((a) - '0') <= 9)
 
 /* This one is more efficient too! ~200 bytes */
 /* In POSIX/C locale (the only locale we care about: do we REALLY want