diff --git a/st/st.c b/st/st.c
index cd1ff7e..cae294e 100644
--- a/st/st.c
+++ b/st/st.c
@@ -422,7 +422,7 @@ static const char base64_digits[] = {
 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 */
 }