diff --git a/src/search.cpp b/src/search.cpp
index 2e3f9d56..3a70c8f7 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -2594,6 +2594,7 @@ namespace {
         // instead of wasting CPU time polling for work.
         while (AllThreadsShouldSleep || threadID >= ActiveThreads)
         {
+            assert(!waitSp);
             assert(threadID != 0);
             threads[threadID].state = THREAD_SLEEPING;