diff --git a/eqpalg/utility/expression_engine.h b/eqpalg/utility/expression_engine.h
index 67c12bd..293aa56 100644
--- a/eqpalg/utility/expression_engine.h
+++ b/eqpalg/utility/expression_engine.h
@@ -11,6 +11,7 @@
 #include <eqpalg/utility/HoldTime.h>
 #include <eqpalg/utility/StatExp.hpp>
 #include <eqpalg/utility/VarsCache.hpp>
+#include <mix_cc/ihyper_db.h>
 #include <mix_cc/matheval/matheval.hpp>
 #include <mix_cc/type/mix_time.h>