diff --git a/simgear/scene/viewer/CompositorUtil.hxx b/simgear/scene/viewer/CompositorUtil.hxx
index d5385908..2b0d47c9 100644
--- a/simgear/scene/viewer/CompositorUtil.hxx
+++ b/simgear/scene/viewer/CompositorUtil.hxx
@@ -27,7 +27,7 @@ namespace compositor {
  * represented in the property tree. Useful for OSG or OpenGL enums.
  */
 template<class T>
-using PropStringMap = const std::unordered_map<std::string, T>;
+using PropStringMap = std::unordered_map<std::string, T>;
 
 template <class T>
 bool findPropString(const std::string &str,