Unable to deposit ether from External Account to Contract

Question

This is my first question on StackOverflow.

I am having trouble sending Ether from an account to the contract. The DApp I am creating is separated into a Data & App contract, where the Data contract is supposed to function as a database.

 pragma solidity ^0.5.8;

 import "./FlightSuretyData.sol";
 import "../node_modules/openzeppelin-solidity/contracts/math/SafeMath.sol";


address private contractOwner;          // Account used to deploy contract

uint constant AIRLINE_REGISTRATION_FEE = 10 ether;

address payable public dataAddress;
FlightSuretyData data;

/**
* @dev Contract constructor
*
*/
constructor (address payable _data, address payable _firstAirline) payable public
{
    contractOwner = msg.sender;
    dataAddress = _data;
    data = FlightSuretyData(_data);

    data.registerAirline("Default", _firstAirline);

}

************************* PROBLEM AREA **********************************
*************************************************************************

/**
* @dev Deposit money to the smart contract
*
*/
function deposit(uint amount) public payable {
   require(msg.value == amount, "Amount not the same!");
   // nothing else to do!

 }

function fund() public payable {

  // make sure enough ether
  require (msg.value >= AIRLINE_REGISTRATION_FEE, "Not enough ether to deposit");

  // transfer money from caller of function to THIS contract
  // PROBLEM
  deposit(msg.value);

  // transfer money from THIS contract to DATA contract
  require(address(this).balance >= AIRLINE_REGISTRATION_FEE, "do not have enough to pay");

  dataAddress.transfer(msg.value);

  require(dataAddress.balance >= AIRLINE_REGISTRATION_FEE, "data contract did not get ether");

  data.fund();
}

  *************************************************************************

I have intentionally given minimal code so that the reader doesn't get sucked into the project details. Also after much debugging, I have figured out the problem is with the deposit function. It simply doesn't deposit (I checked this with the require statement right after).

Indeed, when I run the test below, I get Error: Returned error: VM Exception while processing transaction: revert :

    await config.flightSuretyApp.fund({from: accounts[3], value: 
    web3.utils.toWei("10", "ether")});

Let me know if more code is needed!

function() external payable { fund(); }

flightSuretyData.fund() is: function fund () public { require (airlineDB[tx.origin].isRegistered, "Can only fund registered airlines"); airlineDB[msg.sender].isFunded = true; isAirline[msg.sender] = true; }