diff --git a/src/smt/smt_parallel2.h b/src/smt/smt_parallel2.h
index 99d31a29f..56f229e47 100644
--- a/src/smt/smt_parallel2.h
+++ b/src/smt/smt_parallel2.h
@@ -22,6 +22,7 @@ Revision History:
 #include "util/search_tree.h"
 #include <thread>
 #include <mutex>
+#include <condition_variable>
 
 
 namespace smt {