diff --git a/src/prd.rs b/src/prd.rs
index b1cce62..f4ff01e 100644
--- a/src/prd.rs
+++ b/src/prd.rs
@@ -188,7 +188,7 @@ impl Prd {
         Ok(prd)
     }
 
-    pub fn filter_keys(&mut self, to_keep: HashSet<String>) {
+    pub fn filter_keys(&mut self, to_keep: &HashSet<String>) {
         let current_keys = self.keys.clone();
         let filtered_keys: Map<String, Value> = current_keys
             .into_iter()