diff --git a/src/runtime/object.cpp b/src/runtime/object.cpp
index fab5b3aa32..95dd2758da 100644
--- a/src/runtime/object.cpp
+++ b/src/runtime/object.cpp
@@ -788,6 +788,7 @@ public:
     void resolve(lean_task_object * t, object * v) {
         unique_lock<mutex> lock(m_mutex);
         if (t->m_value) {
+            lock.unlock(); // `dec(v)` could lead to `deactivate_task` trying to take the lock
             dec(v);
             return;
         }