1 files changed, 1 insertions, 1 deletions
diff --git a/base64.c b/base64.c
index 03c6db44..21e1db9e 100644
--- a/base64.c
+++ b/base64.c
@@ -25,7 +25,7 @@ static const char base64val[] = {
     BAD, 26, 27, 28,  29, 30, 31, 32,  33, 34, 35, 36,  37, 38, 39, 40,
      41, 42, 43, 44,  45, 46, 47, 48,  49, 50, 51,BAD, BAD,BAD,BAD,BAD
 };
-#define DECODE64(c)  (isascii(c) ? base64val[c] : BAD)
+#define DECODE64(c)  (isascii((unsigned char)(c)) ? base64val[c] : BAD)
 
 void to64frombits(unsigned char *out, const unsigned char *in, int inlen)
 /* raw bytes in quasi-big-endian order to base 64 string (NUL-terminated) */