diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index b288124049e..83f0fad1f45 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -511,6 +511,7 @@ namespace ts.server.protocol {
         endLocation: Location;
         category: string;
         code: number;
+        source?: string;
         /** May store more in future. For now, this will simply be `true` to indicate when a diagnostic is an unused-identifier diagnostic. */
         reportsUnnecessary?: {};
         reportsDeprecated?: {};