Parse API key from sheet in Powerquery headers

Hello all,

I am querying a REST API in Get & Transform (FKA PowerQuery) in Excel 2016, and I want to parse the API key from a Parameter sheet in Excel.

If I hardcode the API key in the query as part of the headers, all works fine, but if I import it from the sheet and try to use the imported value in the query, I get an authentication failure.

This works:

let
Source = Json.Document(Web.Contents("https://app.url.io/api/", [Headers=[Authorization="Basic APIKEY"]]))
in
Source

This also works:

let
API_key = "APIKEY",
Source = Json.Document(Web.Contents("app.url.io/api/", [Headers=[Authorization="Basic "&API_key]]))
in
Source

And this does not work:

let
//API key from parameter table
API = Excel.CurrentWorkbook(){[Name="API_key"]}[Content],
API_key = API{0}[Column1],
//
Source = Json.Document(Web.Contents("https://app.url.io/api/", [Headers=[Authorization="Basic "&API_key]]))
in
Source

Any ideas on what is going on here?