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Troubleshooting Flow Custom Connectors

This article provides some troubleshooting steps and possible solutions to errors you might see while creating custom connectors in Flow or Power Apps.

Applies to:   Power Automate
Original KB number:   4526409

Custom Connector Test 500 error

Error 500: Expression value is invalid. The template field is required.

Screenshot of the custom connector test 500 error.

There can be several reasons for what might be causing this issue, but the most common reason is related to how the path and host fields are defined in the swagger. The path of each action is relative to the host of the API you're trying to reach, which ends up getting constructed as <host/path>.

For this construction to happen successfully and to properly reach your API's endpoint, the path for the specific action can't be /. You should try having a path with / and a string value afterwards (such as /foo/) instead of just /.


Suppose you want to call the endpoint contoso.com/helloworld. When configuring your custom connector, you might see that the swagger is defined as:

Screenshot shows the invalid swagger when configuring the custom connector.

The problem here's that the host has the name of the endpoint we're trying to call, but the path should be the one with the endpoint name of /helloworld. So the correct way of defining it is:

Screenshot of the correctly configured custom connector.

For more information, see Paths and Operations.