"documentDbPrimaryMasterKey": {
     "type": "object",
     "value": "[listKeys(resourceId('Microsoft.DocumentDB/databaseAccounts', variables('documentDb').name), providers('Microsoft.DocumentDB','databaseAccounts').apiVersions[0]).primaryMasterKey]"
  }

"documentDbPrimaryMasterKey": {
    "type": "String", # <<< STRING
    "value": "[listKeys(resourceId('Microsoft.DocumentDB/databaseAccounts', variables('documentDb').name), providers('Microsoft.DocumentDB','databaseAccounts').apiVersions[0]).primaryMasterKey]"
}