diff --git a/packages/app/lib/modular/index.d.ts b/packages/app/lib/modular/index.d.ts
index 4173586c0c..41e28656cd 100644
--- a/packages/app/lib/modular/index.d.ts
+++ b/packages/app/lib/modular/index.d.ts
@@ -15,13 +15,13 @@ export function deleteApp(app: FirebaseApp): Promise<void>;
  * Registers a library's name and version for platform logging purposes.
  * @param libraryKeyOrName - Library name or key.
  * @param version - Library version.
- * @param variant - Library variant.
+ * @param variant - Library variant. Optional.
  * @returns Promise<void>
  */
 export function registerVersion(
   libraryKeyOrName: string,
   version: string,
-  variant: string | null,
+  variant?: string,
 ): Promise<void>;
 
 /**
diff --git a/packages/app/lib/modular/index.js b/packages/app/lib/modular/index.js
index a5d5c59bfa..f0bdecb9e7 100644
--- a/packages/app/lib/modular/index.js
+++ b/packages/app/lib/modular/index.js
@@ -28,7 +28,7 @@ export function deleteApp(app) {
  * Registers a library's name and version for platform logging purposes.
   @param {string} libraryKeyOrName - library name or key.
   @param {string} version - library version.
-  @param {string | null} variant - library variant.
+  @param {string | undefined} variant - library variant. Optional.
  * @returns {Promise<void>}
  */
 export function registerVersion(libraryKeyOrName, version, variant) {