diff --git a/compound.hh b/compound.hh
index e7304d68d7..99427917d9 100644
--- a/compound.hh
+++ b/compound.hh
@@ -55,7 +55,7 @@ public:
 
     compound_type(compound_type&&) = default;
 
-    auto const& types() {
+    auto const& types() const {
         return _types;
     }