"invalid_request" error on authorization code generation

[williamspaul-1]

Trying to implement OAuth auth code generation, and I'm getting the infamous "invalid_request" message. I can't see any obvious cause -- perhaps a second pair of eyes would be helpful?

The OAuth Policy:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<OAuthV2 async="false" continueOnError="false" enabled="true" name="generate-auth-code">
    <DisplayName>generate-auth-code</DisplayName>
    <ExpiresIn>36000</ExpiresIn>
    <ExternalAuthorization>false</ExternalAuthorization>
    <Operation>GenerateAuthorizationCode</Operation>
    <GenerateResponse enabled="false"/>
    <ResponseType>request.formparam.response_type</ResponseType>
    <ClientId>request.formparam.client_id</ClientId>
    <RedirectUri>request.formparam.redirect_uri</RedirectUri>
</OAuthV2>

The Request (it's in a POST body):

POST /authorizations/authcode HTTP/1.1
Host: myorg-dev.apigee.net
Content-Type: application/x-www-form-urlencoded
Cache-Control: no-cache
Postman-Token: 055899d0-86be-516f-1de7-690d83d99c63

client_id=<app-id>&response_type=code&state=foo

Note that I also tried with and without a variety of other parameters like "grant_type" "scope" "redirect_uri" etc to the same effect.

The Response

{
  "fault": {
    "faultstring": "invalid_request",
    "detail": {
      "errorcode": "invalid_request"
    }
  }
}

I did look at the trace, but the trace also has few details. Is there a way to get a better diagnostic?