diff --git a/spec/ruby/optional/capi/ext/thread_spec.c b/spec/ruby/optional/capi/ext/thread_spec.c
index 845c022fa1..11379bbf2f 100644
--- a/spec/ruby/optional/capi/ext/thread_spec.c
+++ b/spec/ruby/optional/capi/ext/thread_spec.c
@@ -73,19 +73,14 @@ static void* blocking_gvl_func_for_udf_io(void *data) {
   }
 }
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wcast-function-type"
-#endif
-
-static VALUE thread_spec_rb_thread_call_without_gvl_native_function(VALUE self) {
-  pid_t ret = (pid_t) (long) rb_thread_call_without_gvl((void *(*)(void *)) getpid, 0, RUBY_UBF_IO, 0);
-  return LONG2FIX(ret);
+static void* ubf_pid(void *_) {
+  return (void *) (SIGNED_VALUE) getpid();
 }
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
+static VALUE thread_spec_rb_thread_call_without_gvl_native_function(VALUE self) {
+  rb_pid_t ret = (rb_pid_t) (SIGNED_VALUE) rb_thread_call_without_gvl(ubf_pid, 0, RUBY_UBF_IO, 0);
+  return LONG2FIX(ret);
+}
 
 /* Returns true if the thread is interrupted. */
 static VALUE thread_spec_rb_thread_call_without_gvl_with_ubf_io(VALUE self) {