Strange issue creating a custom connector

Hello world,

I'm triyng to achieve my fourth custom connector as I ran in a strange issue. 
While trying to caoncatenate strings I receive this error :
`We cannot convert the value "https://api....." to type Function`

But, using a full string instead of concatenated variables just work. 

Is is the relevant part of the code throwing the error:

api_base_uri =  "https://api.linkedin.com/v2";
[...]
url = if (next_url <> null ) 
      then next_url 
      else api_base_uri & "/organizationPageStatistics?" & Uri.BuildQueryString(  [ 
          q = "organization", 
          organization = organization_urn
      ] )

And the modifed part that just works: 

url = if (next_url <> null ) 
      then next_url 
      else "https://api.linkedin.com/v2" & "/organizationPageStatistics?" & Uri.BuildQueryString(  [ 
          q = "organization", 
          organization = organization_urn
      ] )

Any thought is welcome