diff --git a/libsmtutil/SolverInterface.h b/libsmtutil/SolverInterface.h
index 8dd73b035..4e9cf2258 100644
--- a/libsmtutil/SolverInterface.h
+++ b/libsmtutil/SolverInterface.h
@@ -27,6 +27,7 @@
 #include <cstdio>
 #include <map>
 #include <memory>
+#include <optional>
 #include <string>
 #include <vector>