Dynamodb update javascript sdk with variable in UpdateExpression

let params = {
  TableName: tableName,
  Key: {
    id : id
  },
  UpdateExpression: "SET members[#userId].invitationDate = :invitationDate",
  ConditionExpression: "userId = :userId",
  ExpressionAttributeNames: {
    "#userId": userId,
  },
  ExpressionAttributeValues: {
    ":id": id,
    ":userId": userId,
    ":invitationDate": new Date()
  }
};

Result in

ValidationException: Invalid UpdateExpression: Syntax error; token: ":userId", near: "[#userId]"

Is this impossible with dynamodb? via the AWS.dynamoclient?

via Benoit