diff --git a/src/catch2/internal/catch_optional.hpp b/src/catch2/internal/catch_optional.hpp
index fb7ebbb6..86eca344 100644
--- a/src/catch2/internal/catch_optional.hpp
+++ b/src/catch2/internal/catch_optional.hpp
@@ -8,6 +8,8 @@
 #ifndef CATCH_OPTIONAL_HPP_INCLUDED
 #define CATCH_OPTIONAL_HPP_INCLUDED
 
+#include <cassert>
+
 namespace Catch {
 
     // An optional type