Invalid ssml response when user appears in more than one rota

[sfnash]

Invalid ssml is generated in the response if the User is on-call in multiple rotas:
e.g.
Utterance = "is Joe Bloggs oncall on sunday"

Request:

{
  "session": {
    "sessionId": "XXXXXX",
    "application": {
      "applicationId": "XXXXXX"
    },
    "attributes": {},
    "user": {
      "userId": "XXXXXX"
    },
    "new": true
  },
  "request": {
    "type": "IntentRequest",
    "requestId": "EdwRequestId.XXXXXX",
    "locale": "en-GB",
    "timestamp": "2017-01-17T20:00:20Z",
    "intent": {
      "name": "OnCallIntent",
      "slots": {
        "User": {
          "name": "User",
          "value": "Job Bloggs"
        },
        "Date": {
          "name": "Date",
          "value": "2017-01-22"
        }
      }
    }
  },
  "version": "1.0"
}

Response:

{
  "version": "1.0",
  "response": {
    "outputSpeech": {
      "type": "SSML",
      "ssml": "<speak> <w role=\"ivona:NN\">Joe Bloggs</w> is on call for  indefinitely<w role=\"ivona:NN\">RoleName1 until Monday at 9:00 AM</w>, <w role=\"ivona:1\">RoleName2 until Monday at 9:00 AM</w>. </speak>"
    },
    "shouldEndSession": true
  },
  "sessionAttributes": {}
}

Error: Unable to parse the provided SSML. The provided text is not valid SSML.