diff --git a/src/serialization.cpp b/src/serialization.cpp
index 032ccc4..c305d10 100644
--- a/src/serialization.cpp
+++ b/src/serialization.cpp
@@ -104,6 +104,7 @@ size_t JsonWriteRichPresenceObj(char* dest,
 
         WriteOptionalString(writer, "id", presence->partyId);
         if (presence->partySize) {
+            WriteKey(writer, "size");
             writer.StartArray();
 
             writer.Int(presence->partySize);