diff --git a/bin/xbps-checkvers/main.c b/bin/xbps-checkvers/main.c
index 26c7b231..df8d4585 100644
--- a/bin/xbps-checkvers/main.c
+++ b/bin/xbps-checkvers/main.c
@@ -234,7 +234,7 @@ rcv_sh_substitute(rcv_t *rcv, const char *str, size_t len)
 					cmd = NULL;
 					continue;
 				} else if (*p == '{') {
-					for (ref = ++p; *p && p < str+len && (isalnum(*p) || *p == '_'); p++)
+					for (ref = ++p; *p && p < str+len && (isalnum((unsigned char)*p) || *p == '_'); p++)
 						;
 					reflen = p-ref;
 					switch (*p) {
@@ -253,7 +253,7 @@ rcv_sh_substitute(rcv_t *rcv, const char *str, size_t len)
 						goto err1;
 					}
 				} else {
-					for (ref = p; *p && p < str+len && (isalnum(*p) || *p == '_'); p++)
+					for (ref = p; *p && p < str+len && (isalnum((unsigned char)*p) || *p == '_'); p++)
 						;
 					reflen = p-ref;
 					p--;
@@ -349,7 +349,7 @@ rcv_get_pkgver(rcv_t *rcv)
 				vlen--;
 			}
 			vlen--;
-			while (isspace(v[vlen-1])) {
+			while (isspace((unsigned char)v[vlen-1])) {
 				vlen--;
 			}
 		}
diff --git a/lib/fetch/common.c b/lib/fetch/common.c
index 9a5ef748..f5491e3f 100644
--- a/lib/fetch/common.c
+++ b/lib/fetch/common.c
@@ -927,7 +927,7 @@ fetch_ssl_tolower(char in)
  * conversions.
  */
 static int
-fetch_ssl_isalpha(char in)
+fetch_ssl_isalpha(unsigned char in)
 {
 	return ((in >= 'A' && in <= 'Z') || (in >= 'a' && in <= 'z'));
 }
@@ -964,8 +964,8 @@ fetch_ssl_is_trad_domain_label(const char *l, size_t len, int wcok)
 	if (!len || l[0] == '-' || l[len-1] == '-')
 		return (0);
 	for (i = 0; i < len; ++i) {
-		if (!isdigit(l[i]) &&
-		    !fetch_ssl_isalpha(l[i]) &&
+		if (!isdigit((unsigned char)l[i]) &&
+		    !fetch_ssl_isalpha((unsigned char)l[i]) &&
 		    !(l[i] == '*' && wcok) &&
 		    !(l[i] == '-' && l[i - 1] != '-'))
 			return (0);
diff --git a/lib/util.c b/lib/util.c
index 71afd43b..3de8bf3c 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -55,7 +55,7 @@ static bool is_revision(const char *str) {
 		return false;
 	}
 	/* allow underscore for accepting perl-Digest-1.17_01_1 etc. */
-	while (isdigit(str[0]) || str[0] == '_') {
+	while (isdigit((unsigned char)str[0]) || str[0] == '_') {
 		++str;
 	}
 	return str[0] == '\0';