diff --git a/thread_sync.c b/thread_sync.c
index ef0bbf3af3..bd51328e52 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -272,6 +272,7 @@ rb_mutex_lock(VALUE self)
 	    list_add_tail(&mutex->waitq, &w.node);
 	    native_sleep(th, timeout); /* release GVL */
 	    list_del(&w.node);
+
 	    if (!mutex->th) {
 		mutex->th = th;
 	    }
@@ -287,10 +288,13 @@ rb_mutex_lock(VALUE self)
 		th->status = prev_status;
 	    }
 	    th->vm->sleeper--;
-
 	    if (mutex->th == th) mutex_locked(th, self);
 
-	    RUBY_VM_CHECK_INTS_BLOCKING(th->ec);
+	    RUBY_VM_CHECK_INTS_BLOCKING(th->ec); /* may release mutex */
+	    if (!mutex->th) {
+		mutex->th = th;
+	        mutex_locked(th, self);
+	    }
 	}
     }
     return self;
diff --git a/version.h b/version.h
index 24ae9ad7c0..ef604ffdbf 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.5.2"
 #define RUBY_RELEASE_DATE "2018-10-11"
-#define RUBY_PATCHLEVEL 98
+#define RUBY_PATCHLEVEL 99
 
 #define RUBY_RELEASE_YEAR 2018
 #define RUBY_RELEASE_MONTH 10