st.c | 2 +-

diff --git a/st.c b/st.c
index c71fa0677ee40d7ba71b5b67fbb70b4386500be8..1307fdf4ab3afee8d16af827b1a04a99152cf1ec 100644
--- a/st.c
+++ b/st.c
@@ -367,7 +367,7 @@
 char
 base64dec_getc(const char **src)
 {
-	while (**src && !isprint(**src))
+	while (**src && !isprint((unsigned char)**src))
 		(*src)++;
 	return **src ? *((*src)++) : '=';  /* emulate padding if string ends */
 }