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Power Automate "InvalidTemplate": why an expression fails and how to fix it

You see: InvalidTemplate ยท Unable to process template language expressions in action

What it means

An expression in one of the flow's actions could not be worked out. The input it points at is empty, missing or the wrong type, for example reading a field from an item that does not have it, or taking the first item of an empty list.

How to fix it

  1. Open the flow's run history, open the failed run and click the action marked with the red cross. The error names the action and the expression.
  2. Open the earlier action the expression reads from and look at its outputs: is the field there for this run, and is it text, a number or a list?
  3. Make the expression forgiving: read optional fields with the question-mark form, body('Get_item')?['Title'], and give empty values a fallback with coalesce(), for example coalesce(triggerBody()?['email'], '').
  4. Check lists before using them: use a Condition on length(body('List_rows')?['value']) greater than 0 before first(), or use a loop.
  5. Save, then resubmit the failed run from run history.

If it keeps coming back

If it fails only for certain runs, the trigger is sometimes sent without a field. Test with that run's data on the trigger and add the fallback there.

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