diff --git a/src/builtin.c b/src/builtin.c
index 72357a9fc9..a5d59fbfa6 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -1282,7 +1282,12 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
   const char *fmt = jv_string_value(b);
   const char *end = strptime(input, fmt, &tm);
 
-  if (end == NULL || (*end != '\0' && !isspace(*end))) {
+  /*
+   * "*end" may be a signed char, and passing a value which is less than -1
+   * to isspace() violates the C standard, thus, casting to (unsigned char *)
+   * is necessary.
+   */
+  if (end == NULL || (*end != '\0' && !isspace(*(unsigned char *)end))) {
     jv e = jv_invalid_with_msg(jv_string_fmt("date \"%s\" does not match format \"%s\"", input, fmt));
     jv_free(a);
     jv_free(b);
diff --git a/src/main.c b/src/main.c
index a7f50b5135..74ecb08a38 100644
--- a/src/main.c
+++ b/src/main.c
@@ -102,8 +102,13 @@ static void die() {
 
 
 
+/*
+ * "text[1]" may be a signed char, and passing a value which is less than -1
+ * to isalpha() violates the C standard, thus, casting to (unsigned char *)
+ * is necessary.
+ */
 static int isoptish(const char* text) {
-  return text[0] == '-' && (text[1] == '-' || isalpha(text[1]));
+  return text[0] == '-' && (text[1] == '-' || isalpha(((const unsigned char *)text)[1]));
 }
 
 static int isoption(const char* text, char shortopt, const char* longopt, size_t *short_opts) {