diff --git a/py/modthread.c b/py/modthread.c
index d0e71dad3..1d7602789 100644
--- a/py/modthread.c
+++ b/py/modthread.c
@@ -84,7 +84,9 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(thread_lock_acquire_obj, 1, 3, thread
 
 STATIC mp_obj_t thread_lock_release(mp_obj_t self_in) {
     mp_obj_thread_lock_t *self = MP_OBJ_TO_PTR(self_in);
-    // TODO check if already unlocked
+    if (!self->locked) {
+        mp_raise_msg(&mp_type_RuntimeError, NULL);
+    }
     self->locked = false;
     MP_THREAD_GIL_EXIT();
     mp_thread_mutex_unlock(&self->mutex);
diff --git a/tests/thread/thread_lock1.py b/tests/thread/thread_lock1.py
index ca585ffbb..ba5c7dff0 100644
--- a/tests/thread/thread_lock1.py
+++ b/tests/thread/thread_lock1.py
@@ -38,3 +38,9 @@ try:
 except KeyError:
     print('KeyError')
     print(lock.locked())
+
+# test that we can't release an unlocked lock
+try:
+    lock.release()
+except RuntimeError:
+    print('RuntimeError')