Template error: round got invalid input 'unknown' when rendering template #25

Hi,

I just imported the Weather forecast blueprint but it wont run.
It keeps throwing me this error :
Error rendering variables: ValueError: Template error: round got invalid input 'unknown' when rendering template '{{states(current_temp_var) | round(temp_digits)}}' but no default was specified
Error rendering variables: TypeError: '>' not supported between instances of 'NoneType' and 'int'

I tried using different temperature sensors to see if this fixes the issue but it always fails on this step. Not sure how to fix or continue ?