diff --git a/src/builtin.c b/src/builtin.c
index 7d21bfb111..e39975b0a0 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -1474,7 +1474,7 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
   }
 #endif
   const char *end = strptime(input, fmt, &tm);
-  if (end == NULL || (*end != '\0' && !isspace(*end))) {
+  if (end == NULL || (*end != '\0' && !isspace((unsigned char)*end))) {
     return ret_error2(a, b, jv_string_fmt("date \"%s\" does not match format \"%s\"", input, fmt));
   }
   jv_free(b);
diff --git a/src/main.c b/src/main.c
index 2f26ff2bfe..a8c217fcda 100644
--- a/src/main.c
+++ b/src/main.c
@@ -126,7 +126,7 @@ static void die() {
 }
 
 static int isoptish(const char* text) {
-  return text[0] == '-' && (text[1] == '-' || isalpha(text[1]));
+  return text[0] == '-' && (text[1] == '-' || isalpha((unsigned char)text[1]));
 }
 
 static int isoption(const char* text, char shortopt, const char* longopt, size_t *short_opts) {