1 files changed, 4 insertions, 3 deletions
diff --git a/csharp/src/Google.Protobuf/Collections/MapField.cs b/csharp/src/Google.Protobuf/Collections/MapField.cs
index 90a5ff1a..993a89d7 100644
--- a/csharp/src/Google.Protobuf/Collections/MapField.cs
+++ b/csharp/src/Google.Protobuf/Collections/MapField.cs
@@ -34,6 +34,7 @@ using Google.Protobuf.Reflection;
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Text;
 using Google.Protobuf.Compatibility;
@@ -474,9 +475,9 @@ namespace Google.Protobuf.Collections
         /// </summary>
         public override string ToString()
         {
-            var builder = new StringBuilder();
-            JsonFormatter.Default.WriteDictionary(builder, this);
-            return builder.ToString();
+            var writer = new StringWriter();
+            JsonFormatter.Default.WriteDictionary(writer, this);
+            return writer.ToString();
         }
 
         #region IDictionary explicit interface implementation