Error while pushing from bitbucket repo to heroku. It says my repo is a shallow clone

I created a pipeline to deploy my code from Bitbucket repo to Heroku server.

my bitbucket-pipelines.yml

image: node:10.15.3

pipelines:
  default:
    - step:
        script: # Modify the commands below to build your repository.
          - npm install
          - git push https://heroku:<my heroku api key>@git.heroku.com/<my heroku app's name>.git HEAD

After pushing my code to my bitbucket repo, the pipeline runs, but fails while pushing the code to heroku git. Below is the error:

Push rejected, source repository is a shallow clone. Unshallow it with `git fetch --all --unshallow` and try pushing again.

But then by bit bucket repo is not shallow. The command git rev-parse --is-shallow-repository returns false.