Using multi select field in pipelines

I had the same issue, I was able to bypass this by adding jinja to the multi-select field in pipelines. In order to start the JINJA2 process for a multi-select field you must first drag and drop from the list of the table you desire a field into the multi select field. Once that happens, you can then create Jinja2 code. This was my code to fix the error you are speaking of:

B.Field is my multi-select field on Table B that will be receiving data
A.Text is my text field on Table A 

{% if B.Field == none %}
  {{ A.Text }}
{% elif B.Field != none %}
  {{ [A.Text, B.Field]|join(' ;') }}
{% endif %}

Hope this helps!