diff --git a/server/src/session.ts b/server/src/session.ts
index 17b81de63c..cbeeb9bcf3 100644
--- a/server/src/session.ts
+++ b/server/src/session.ts
@@ -708,8 +708,12 @@ export class Session {
       this.logger.info(`Failed to get language service for closed project ${project.projectName}.`);
       return undefined;
     }
+    const languageService = project.getLanguageService();
+    if (!isNgLs(languageService)) {
+      return undefined;
+    }
     return {
-      languageService: project.getLanguageService() as NgLanguageService,
+      languageService,
       scriptInfo,
     };
   }
@@ -937,3 +941,7 @@ function toArray<T>(it: ts.Iterator<T>): T[] {
   }
   return results;
 }
+
+function isNgLs(ls: ts.LanguageService|NgLanguageService): ls is NgLanguageService {
+  return 'getTcb' in ls;
+}
\ No newline at end of file