diff --git a/src/vs/workbench/parts/debug/common/debugProtocol.d.ts b/src/vs/workbench/parts/debug/common/debugProtocol.d.ts
index 92471938bb76b6c6d0d95938637c0c3e662c4be6..cc1ff5ee5bab5a07c242e1fb8ac0b8da66f5e8bf 100644
--- a/src/vs/workbench/parts/debug/common/debugProtocol.d.ts
+++ b/src/vs/workbench/parts/debug/common/debugProtocol.d.ts
@@ -732,7 +732,9 @@ declare module DebugProtocol {
 
 	/** Arguments for 'source' request. */
 	export interface SourceArguments {
-		/** The reference to the source. This is the value received in Source.reference. */
+		/** The source: either source.path or source.sourceReference must be specified. */
+		source?: Source;
+		/** For backward compatibility: The reference to the source. This is the value received in Source.reference. */
 		sourceReference: number;
 	}