diff --git a/ext/json/ext/parser/parser.c b/ext/json/ext/parser/parser.c
index 297031dc..2522c562 100644
--- a/ext/json/ext/parser/parser.c
+++ b/ext/json/ext/parser/parser.c
@@ -124,7 +124,7 @@ static VALUE rstring_cache_fetch(rvalue_cache *cache, const char *str, const lon
         return Qfalse;
     }
 
-    if (RB_UNLIKELY(!isalpha((unsigned char)str[0]))) {
+    if (RB_UNLIKELY(!rb_isalpha((unsigned char)str[0]))) {
         // Simple heuristic, if the first character isn't a letter,
         // we're much less likely to see this string again.
         // We mostly want to cache strings that are likely to be repeated.
@@ -176,7 +176,7 @@ static VALUE rsymbol_cache_fetch(rvalue_cache *cache, const char *str, const lon
         return Qfalse;
     }
 
-    if (RB_UNLIKELY(!isalpha((unsigned char)str[0]))) {
+    if (RB_UNLIKELY(!rb_isalpha((unsigned char)str[0]))) {
         // Simple heuristic, if the first character isn't a letter,
         // we're much less likely to see this string again.
         // We mostly want to cache strings that are likely to be repeated.