The “Block length does not match with its complement” Error In Power Query In Power BI And Excel

When using the Web.Contents function to call a web service in Power Query in either Power BI or Excel you may encounter the following error:

DataFormat.Error: Block length does not match with its complement.

Here’s an example of an M query that calls a web service and, at the time of writing, gives this error:

Web.Contents(
  "https://api.open-meteo.com/v1/forecast?latitude=52.52&longitude=13.41&hourly=temperature_2m"
)

The problem is caused by something to do with how some web services handle “deflate” compression; apparently the same issue exists in the equivalent native .NET functionality. There are two ways to avoid it though.

First of all, the easy way: ask the web service to use “gzip” compression instead. You can do this by setting the “Accept-Encoding” header to “gzip” like so:

Web.Contents(
  "https://api.open-meteo.com/v1/forecast?latitude=52.52&longitude=13.41&hourly=temperature_2m",
  [
    Headers = [
      #"Accept-Encoding" = "gzip"
    ]
  ]
)

Having done this the error will no longer occur and you can go on to handle the data returned by the web service (which is a JSON document) as normal.

The second is to use the Web.BrowserContents function instead of Web.Contents:

Web.BrowserContents(
  "https://api.open-meteo.com/v1/forecast?latitude=52.52&longitude=13.41&hourly=temperature_2m"
)

This is more complicated because the Web.BrowserContents function returns the HTML for a web page as viewed by a web browser. Apart from issues like handling authentication you’ll need to parse the result to get the data you need:

It’s doable using the Html.Table function though. Here’s an example of how to handle the response for this particular web service call:

let
  Source = Web.BrowserContents(
    "https://api.open-meteo.com/v1/forecast?latitude=52.52&longitude=13.41&hourly=temperature_2m"
  ),
  HTMLTable = Html.Table(
    Source,
    {{"JSONPayload", "pre"}}
  ),
  JSON = Json.Document(
    HTMLTable{0}[JSONPayload]
  )
in
  JSON

[Thanks to Thais Marshall for bringing me this problem, and to Curt Hagenlocher for explaining and it showing me the first workaround]