diff --git a/src/library/mt_task_queue.cpp b/src/library/mt_task_queue.cpp
index 263566bfb9..5a5485c504 100644
--- a/src/library/mt_task_queue.cpp
+++ b/src/library/mt_task_queue.cpp
@@ -165,6 +165,10 @@ void mt_task_queue::handle_finished(gtask const & t) {
                 // TODO(gabriel): removed failed tasks from reverse dependency lists?
                 m_waiting.erase(rdep);
                 break;
+            case task_state::Success:
+                // this can happen if a task occurs in more than one reverse dependency list,
+                // or gets submitted more than once
+                break;
             default: lean_unreachable();
         }
     }