solc command is generating empty .bin files

Question

Firstly, this is not an inheritance problem like in this post

I have the following two files ( ManagedWallet.sol & ManagedWalletLookup.sol )

pragma solidity ^0.4.24;

contract ManagedWalletLookup { 
    address private _owner;

    mapping(address => address) public _managedWallets;

    constructor () internal {
        _owner = msg.sender;
    }

    modifier isOwner() {
        require(msg.sender == _owner);
        _;
    }

    function updateManagedWallets(address wallet, address owner) public 
        isOwner {
        _managedWallets[owner] = wallet;
    }

}

and...

pragma solidity ^0.4.24;

contract ManagedWallet {
    address private _owner;
    address private _manager;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    constructor (address owner) internal {
        _owner = owner;
        _manager = msg.sender;
    }

    modifier isOwnerOrManager() {
        require(msg.sender == _owner || msg.sender == _manager);
        _;
    }

    function transferOwnership(address newOwner) public isOwnerOrManager {
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

When I type

 solc --version 

into the terminal it prints

Version: 0.4.24+commit

so the solc is the right version. However, anytime I enter the command

solc ManagedWallet.sol --bin --abi --optimize -o ./

or

solc ManagedWalletLookup.sol --bin --abi --optimize -o ./

It returns empty .bin files

Any idea why this might be happening?