diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index 0ba0e6e..53cab6d 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -869,6 +869,7 @@ VALUE rb_mutex_lock(VALUE mutex);
 VALUE rb_mutex_unlock(VALUE mutex);
 VALUE rb_mutex_sleep(VALUE self, VALUE timeout);
 VALUE rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg);
+VALUE rb_obj_is_mutex(VALUE obj);
 /* time.c */
 VALUE rb_time_new(time_t, long);
 VALUE rb_time_nano_new(time_t, long);
diff --git a/internal.h b/internal.h
index 795749d..07e46b9 100644
--- a/internal.h
+++ b/internal.h
@@ -620,7 +620,6 @@ VALUE rb_struct_init_copy(VALUE copy, VALUE s);
 struct timeval rb_time_timeval(VALUE);
 
 /* thread.c */
-VALUE rb_obj_is_mutex(VALUE obj);
 VALUE rb_suppress_tracing(VALUE (*func)(VALUE), VALUE arg);
 void rb_thread_execute_interrupts(VALUE th);
 void rb_clear_trace_func(void);