diff --git a/extensions/json/server/src/languageModelCache.ts b/extensions/json/server/src/languageModelCache.ts
index b12f77b52a6..7496244c3bb 100644
--- a/extensions/json/server/src/languageModelCache.ts
+++ b/extensions/json/server/src/languageModelCache.ts
@@ -16,7 +16,7 @@ export function getLanguageModelCache<T>(maxEntries: number, cleanupIntervalTime
 	let languageModels: { [uri: string]: { version: number, languageId: string, cTime: number, languageModel: T } } = {};
 	let nModels = 0;
 
-	let cleanupInterval = void 0;
+	let cleanupInterval: NodeJS.Timer | undefined = void 0;
 	if (cleanupIntervalTimeInSec > 0) {
 		cleanupInterval = setInterval(() => {
 			let cutoffTime = Date.now() - cleanupIntervalTimeInSec * 1000;