diff --git a/src/buffers.c b/src/buffers.c
index 79cc45bf5..2557fe4a0 100644
--- a/src/buffers.c
+++ b/src/buffers.c
@@ -407,7 +407,7 @@ int chunk_htmlencode(struct chunk *dst, struct chunk *src) {
 
 		c = src->str[i];
 
-		if (!isascii(c) || !isprint(c) || c == '&' || c == '"' || c == '\'' || c == '<' || c == '>') {
+		if (!isascii(c) || !isprint((unsigned char)c) || c == '&' || c == '"' || c == '\'' || c == '<' || c == '>') {
 			l = snprintf(dst->str + dst->len, free, "&#%u;", (unsigned char)c);
 
 			if (free < l) {
@@ -447,7 +447,7 @@ int chunk_asciiencode(struct chunk *dst, struct chunk *src, char qc) {
 
 		c = src->str[i];
 
-		if (!isascii(c) || !isprint(c) || c == '<' || c == '>' || c == qc) {
+		if (!isascii(c) || !isprint((unsigned char)c) || c == '<' || c == '>' || c == qc) {
 			l = snprintf(dst->str + dst->len, free, "<%02X>", (unsigned char)c);
 
 			if (free < l) {
diff --git a/src/cfgparse.c b/src/cfgparse.c
index b5437a6a1..147f0185b 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -727,7 +727,8 @@ int cfg_parse_global(const char *file, int linenum, char **args, int kwm)
 
 		for (i=0; args[1][i]; i++) {
 			c = args[1][i];
-			if (!isupper(c) && !islower(c) && !isdigit(c) && c != '_' && c != '-' && c != '.')
+			if (!isupper((unsigned char)c) && !islower((unsigned char)c) &&
+			    !isdigit((unsigned char)c) && c != '_' && c != '-' && c != '.')
 				break;
 		}
 
@@ -2506,7 +2507,8 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int kwm)
 
 				for (i=0; args[2][i]; i++) {
 					c = args[2][i];
-					if (!isupper(c) && !islower(c) && !isdigit(c) && c != '_' && c != '-' && c != '.')
+					if (!isupper((unsigned char)c) && !islower((unsigned char)c) &&
+					    !isdigit((unsigned char)c) && c != '_' && c != '-' && c != '.')
 						break;
 				}
 
diff --git a/src/checks.c b/src/checks.c
index b235d9e15..a023bf7fd 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -906,7 +906,8 @@ static int event_srv_chk_r(int fd)
 		if ((len < strlen("HTTP/1.0 000\r")) ||
 		    (memcmp(trash, "HTTP/1.", 7) != 0 ||
 		    (trash[12] != ' ' && trash[12] != '\r')) ||
-		    !isdigit(trash[9]) || !isdigit(trash[10]) || !isdigit(trash[11])) {
+		    !isdigit((unsigned char)trash[9]) || !isdigit((unsigned char)trash[10]) ||
+		    !isdigit((unsigned char)trash[11])) {
 
 			cut_crlf(trash);
 			set_server_check_status(s, HCHK_STATUS_L7RSP, trash);
@@ -941,7 +942,8 @@ static int event_srv_chk_r(int fd)
 		/* Check if the server speaks SMTP */
 		if ((len < strlen("000\r")) ||
 		    (trash[3] != ' ' && trash[3] != '\r') ||
-		    !isdigit(trash[0]) || !isdigit(trash[1]) || !isdigit(trash[2])) {
+		    !isdigit((unsigned char)trash[0]) || !isdigit((unsigned char)trash[1]) ||
+		    !isdigit((unsigned char)trash[2])) {
 
 			cut_crlf(trash);
 			set_server_check_status(s, HCHK_STATUS_L7RSP, trash);
diff --git a/src/standard.c b/src/standard.c
index 2e6893654..810672593 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -184,7 +184,7 @@ const char *invalid_char(const char *name)
 		return name;
 
 	while (*name) {
-		if (!isalnum((int)*name) && *name != '.' && *name != ':' &&
+		if (!isalnum((int)(unsigned char)*name) && *name != '.' && *name != ':' &&
 		    *name != '_' && *name != '-')
 			return name;
 		name++;
@@ -203,7 +203,7 @@ const char *invalid_domainchar(const char *name) {
 		return name;
 
 	while (*name) {
-		if (!isalnum((int)*name) && *name != '.' &&
+		if (!isalnum((int)(unsigned char)*name) && *name != '.' &&
 		    *name != '_' && *name != '-')
 			return name;