diff --git a/rust/nix-expr/src/eval_state.rs b/rust/nix-expr/src/eval_state.rs
index 51e236c..ec1b818 100644
--- a/rust/nix-expr/src/eval_state.rs
+++ b/rust/nix-expr/src/eval_state.rs
@@ -175,6 +175,11 @@ impl EvalState {
             paths
         };
 
+        // We've converted the nix_realised_string to a native struct containing copies, so we can free it now.
+        unsafe {
+            raw::realised_string_free(rs);
+        }
+
         Ok(RealisedString { s, paths })
     }