Flowdock: REST API - Getting an error: "nil is not a supported event"

Document ID : KB000105837
Last Modified Date : 30/07/2018
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When sending a message to my flow using REST API, getting an error: "nil is not a supported event".

Here is what we're doing:
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This results in this 400 Bad Request status code and this error message:

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The reason for this error is your input raw data format is specified as 'text'. It needs to be changed to 'JSON'.

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This will result successfully:

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Additional Information:
Flowdock's help: https://www.flowdock.com/help

Flowdock's API: https://www.flowdock.com/api