diff --git a/src/Set.h b/src/Set.h
index febb05edc..2e077ccb1 100644
--- a/src/Set.h
+++ b/src/Set.h
@@ -93,7 +93,7 @@ template<class T, class U> class Set {
         }
 
         /** @brief Value in underlying type */
-        inline constexpr UnderlyingType toUnderlyingType() const {
+        inline constexpr explicit operator UnderlyingType() const {
             return value;
         }