diff --git a/src/value/ser.rs b/src/value/ser.rs
index a68193437..1ffe7b7f1 100644
--- a/src/value/ser.rs
+++ b/src/value/ser.rs
@@ -261,9 +261,9 @@ impl serde::Serializer for Serializer {
         })
     }
 
-    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap> {
+    fn serialize_map(self, len: Option<usize>) -> Result<Self::SerializeMap> {
         Ok(SerializeMap::Map {
-            map: Map::new(),
+            map: Map::with_capacity(len.unwrap_or(0)),
             next_key: None,
         })
     }