Contract method works only with contract addresses and not regular account addresses

Question

I've written a simple token contract for testing purposes:

pragma solidity ^0.4.18;

contract TestToken {
    uint public totalSupply;

    mapping(address => uint) public balanceOf;

    function transfer(address to, uint amount) public returns (bool) {
        require(balanceOf[msg.sender] >= amount);
        balanceOf[msg.sender] -= amount;
        balanceOf[to] += amount;
        return true;
    }

    function credit(address to, uint amount) public {
        balanceOf[to] += amount;
        totalSupply += amount;
    }

    function debit(address from, uint amount) public {
        balanceOf[from] -= amount;
        totalSupply -= amount;
    }
}

Whenever I call the credit method on the contract and pass the address of another contract as parameter, everything works as expected:

token.methods.credit(wallet._address, 500000).send({from: accounts[0], gas: "0xB3E4", gasPrice: "0x4A817C800", value: "0x"})

The status of the transaction is 0x1.

The balance is updated:

token.methods.balanceOf(wallet._address).call()
'500000'

But when I pass the address of a regular ethereum account as a parameter, the transaction gets mined, but nothing happens.

token.methods.credit(accounts[0], 500000).send({from: accounts[0], gas: "0xB3E4", gasPrice: "0x4A817C800", value: "0x"})

The status of the transaction is 0x0.

The balance doesn't change:

token.methods.balanceOf(accounts[0]).call()
'0'

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Comparing the resulting signed transaction objects, the only difference other than the data field is that the signature v value is always 0 when the transaction fails and 1 when the transaction succeeds. But I'm not sure what that means.

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Update:

Seems like it's not even contract vs non-contract, it's just that some addresses don't work.

For example, this works:

token.methods.credit('0xd2d0aD819B2679FBDD5C75F93fa13242bfd7E2A5', 500000000000000000).send({from: accounts[0], gas: "0xB3E4", gasPrice: "0x4A817C800", value: "0x"})

This doesn't:

token.methods.credit('0xc4beccd2ebdb32203800dfbc60ff0dd7c2762b48', 500000000000000000).send({from: accounts[0], gas: "0xB3E4", gasPrice: "0x4A817C800", value: "0x"})

Answer 1

The problem is most probably with gas limit. How did you calculate it?

When transaction has some data attached, e.g. contract's method selector and parameters, zero bytes cost less than non-zero. So, transaction that sends tokens to 0x0011223344556677889900112233445566778899 will cost less than transaction, that send the same number of tokens to 0x112233445566778899aa112233445566778899aa. Also, changing value in storage from zero to non-zero costs more, than changing it from non-zero to non-zero. So balanceOf[to] += amount; costs less if to address already has some tokens.

Answer 2

1. you're sending from account[0] to account[0], so the balance stays he same
2. if the balance of (accounts[0]) is zero, as you show in your second call, then transfer call will fail because of require(balanceOf[msg.sender] >= amount)