diff --git a/src/tactic/tactical.cpp b/src/tactic/tactical.cpp
index c3152ed58..ca66b7a14 100644
--- a/src/tactic/tactical.cpp
+++ b/src/tactic/tactical.cpp
@@ -19,6 +19,7 @@ Notes:
 #include"tactical.h"
 #include"scoped_timer.h"
 #include"cancel_eh.h"
+#include"cooperate.h"
 #include"scoped_ptr_vector.h"
 #include"z3_omp.h"