diff --git a/src/tactic/sls_tactic/sls_tactic.cpp b/src/tactic/sls_tactic/sls_tactic.cpp
index 93e61f3b3..6765ef4cb 100644
--- a/src/tactic/sls_tactic/sls_tactic.cpp
+++ b/src/tactic/sls_tactic/sls_tactic.cpp
@@ -34,6 +34,7 @@ Notes:
 #include"expr2dot.h"
 #include"propagate_values_tactic.h"
 #include"sls_tactic.h"
+#include"nnf_tactic.h"
 
 class sls_tactic : public tactic {
     class stats {