diff --git a/src/data/repositories/auth_repository.ts b/src/data/repositories/auth_repository.ts
index 171976ce..e33dfc6e 100644
--- a/src/data/repositories/auth_repository.ts
+++ b/src/data/repositories/auth_repository.ts
@@ -70,11 +70,12 @@ export class AuthRepository implements IAuthRepository {
     guestId?: string;
   }): Promise<Result<LoginResponse>> {
     return Result.wrap(async () => {
+      const email = input.email?.trim() ?? "";
       const response = await this.api.emailLogin(
         LoginRequest.from(
           withTestAccountFlag({
-            email: input.email ?? "",
-            username: input.username ?? "",
+            email,
+            username: resolveLoginUsername(input.username, email),
             password: input.password,
             platform: getAuthPlatform(),
             guestId: input.guestId ?? "",
@@ -332,3 +333,11 @@ function withTestAccountFlag<T extends Record<string, unknown>>(
 function getAuthPlatform(): string {
   return PlatformDetector.getPlatform();
 }
+
+function resolveLoginUsername(
+  username: string | undefined,
+  email: string,
+): string {
+  const trimmedUsername = username?.trim() ?? "";
+  return trimmedUsername.length > 0 ? trimmedUsername : email;
+}