diff --git a/dev/src/write-batch.ts b/dev/src/write-batch.ts
index d567ee1e3..1e6ee5bb5 100644
--- a/dev/src/write-batch.ts
+++ b/dev/src/write-batch.ts
@@ -728,7 +728,7 @@ function validateUpdatePrecondition(
   arg: string | number,
   value: unknown,
   options?: RequiredArgumentOptions
-): void {
+): asserts value is {lastUpdateTime?: Timestamp} {
   if (!validateOptional(value, options)) {
     validatePrecondition(arg, value, /* allowExists= */ false);
   }