Gateways - Update Datasource request fails: Invalid Ciphertext size.

When I make a request to update datasource for a gateway, I get an error about ciphertext that seems rather confusing since I'm just using Basic credentials. Any idea what this means?

PATCH https://api.powerbi.com/v1.0/myorg/gateways/(my-gateway-id)/datasources/(my-data-set-id)

{
  "credentialDetails": {
    "credentialType": "Basic",
    "credentials": "{\"credentialData\":[{\"name\":\"username\", \"value\":\"my_username\"},{\"name\":\"password\", \"value\":\"my_password\"}]}",
    "encryptedConnection": "NotEncrypted",
    "encryptionAlgorithm": "None",
    "privacyLevel": "None"
  }
}

{
  "error": {
    "code": "DM_GWPipeline_UnknownError",
    "pbi.error": {
      "code": "DM_GWPipeline_UnknownError",
      "parameters": {},
      "details": [
        {
          "code": "DM_ErrorDetailNameCode_UnderlyingErrorMessage",
          "detail": {
            "type": 1,
            "value": "Invalid Ciphertext size. Expecting cipher text of atleast length, 344. Provided cipherText length, 93"
          }
        },
        {
          "code": "DM_ErrorDetailNameCode_UnderlyingHResult",
          "detail": {
            "type": 1,
            "value": "-2146233296"
          }
        }
      ],
      "exceptionCulprit": 1
    }
  }
}