found a tab character that violate intendation while scanning a plain scalar at line 20 column 7

Summary

There is no tab characters, but error occurred while building tags, it works while building master branch

Found errors in your .gitlab-ci.yml: (): found a tab character that violate intendation while scanning a plain scalar at line 20 column 7

This is my .gitlab-ci.yml

image: jk2k/gitlab-ci-android-with-ndk

before_script:
  - export GRADLE_USER_HOME=/cache/.gradle
  - export ANDROID_NDK_HOME=/sdk/android-ndk-r12b

stages:
  - build

debug:
  stage: build
  only:
    - master
  script:
    - ./gradlew assembleDebug

release:
  stage: build
  only:
    - /^v.*$/
    - tags
    - triggers
  script:
    - ./gradlew assembleRelease
    - ./fir-publisher
  artifacts:
    paths:
    - app/build/outputs/apk/app-release.apk

(Summarize the bug encountered concisely)

Steps to reproduce

error occurred while building tags, it works while building master branch

Expected behavior

Actual behavior

Relevant logs and/or screenshots

Version: GitLab 8.11.5