diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3d190f144ad..4c751c7048c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
     - NodeJS:
       - CHANGED: Use node-api instead of NAN. [#6452](https://github.com/Project-OSRM/osrm-backend/pull/6452)
     - Misc:
+      - FIXED: Fix an error in a RouteParameters AnnotationsType operator overload. [#6646](https://github.com/Project-OSRM/osrm-backend/pull/6646)
       - ADDED: Add support for "unlimited" to be passed as a value for the default-radius and max-matching-radius flags. [#6599](https://github.com/Project-OSRM/osrm-backend/pull/6599)
       - CHANGED: Allow -1.0 as unlimited for default_radius value. [#6599](https://github.com/Project-OSRM/osrm-backend/pull/6599)
       - CHANGED: keep libosrm* in the docker image for downstream linking [#6602](https://github.com/Project-OSRM/osrm-backend/pull/6602)
diff --git a/include/engine/api/route_parameters.hpp b/include/engine/api/route_parameters.hpp
index fe121a06d1a..3593ecf5ba5 100644
--- a/include/engine/api/route_parameters.hpp
+++ b/include/engine/api/route_parameters.hpp
@@ -202,8 +202,8 @@ inline RouteParameters::AnnotationsType operator|(RouteParameters::AnnotationsTy
         static_cast<std::underlying_type_t<RouteParameters::AnnotationsType>>(rhs));
 }
 
-inline RouteParameters::AnnotationsType operator|=(RouteParameters::AnnotationsType lhs,
-                                                   RouteParameters::AnnotationsType rhs)
+inline RouteParameters::AnnotationsType &operator|=(RouteParameters::AnnotationsType &lhs,
+                                                    RouteParameters::AnnotationsType rhs)
 {
     return lhs = lhs | rhs;
 }