diff --git a/mjit.c b/mjit.c
index 10aa15fb7f..e166a066bf 100644
--- a/mjit.c
+++ b/mjit.c
@@ -120,7 +120,7 @@ extern void rb_native_cond_wait(rb_nativethread_cond_t *cond, rb_nativethread_lo
 extern int rb_thread_create_mjit_thread(void (*child_hook)(void), void (*worker_func)(void));
 
 /* process.c */
-pid_t ruby_waitpid_locked(rb_vm_t *, rb_pid_t, int *status, int options,
+rb_pid_t ruby_waitpid_locked(rb_vm_t *, rb_pid_t, int *status, int options,
                           rb_nativethread_cond_t *cond);
 
 #define RB_CONDATTR_CLOCK_MONOTONIC 1
diff --git a/process.c b/process.c
index 0c967538d3..dbdcade71a 100644
--- a/process.c
+++ b/process.c
@@ -971,7 +971,7 @@ waitpid_state_init(struct waitpid_state *w, pid_t pid, int options)
 /*
  * must be called with vm->waitpid_lock held, this is not interruptible
  */
-pid_t
+rb_pid_t
 ruby_waitpid_locked(rb_vm_t *vm, rb_pid_t pid, int *status, int options,
                     rb_nativethread_cond_t *cond)
 {