[Ethereum] Uncaught Error: could not decode, json: cannot unmarshal array into Go value of type string(…)

I'm trying to use my already deployed contract using web3.js from my console but i'm having issues.

I get my compiled contract from browser-solidity then do:

var web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"));
var MyContract = web3.eth.contract(abi);
var myContractInstance = MyContract.at('address');
web3.eth.getCode(address)  => returns my contract code

then, if I type myContractinstance, I get the list of methods that I've built in solidity.

Issue is: I'm getting this error for all of the function calls.

Uncaught Error: could not decode, json: cannot unmarshal array into Go value of type string

Anyone got any idea of what could be happening ?

Edit:
Sorry for late edit

I'm using a slightly modified version of that standardized contract :

contract currency {

                struct Account {
                    uint balance;
                    mapping ( address => uint) withdrawers;
                }

                mapping ( address => Account ) accounts;

                event CoinSent(address indexed from, uint256 value, address indexed to);

                function currency() {
                    accounts[msg.sender].balance = 1000000;
                }

                function sendCoin(uint _value, address _to) returns (bool _success) {
                    if (accounts[msg.sender].balance >= _value && _value < 340282366920938463463374607431768211456) {
                        accounts[msg.sender].balance -= _value;
                        accounts[_to].balance += _value;
                        CoinSent(msg.sender, _value, _to);
                        _success = true;
                    }
                    else _success = false;
                }

                function sendCoinFrom(address _from, uint _value, address _to) returns (bool _success) {
                    uint auth = accounts[_from].withdrawers[msg.sender];
                    if (accounts[_from].balance >= _value && auth >= _value && _value < 340282366920938463463374607431768211456) {
                        accounts[_from].withdrawers[msg.sender] -= _value;
                        accounts[_from].balance -= _value;
                        accounts[_to].balance += _value;
                        CoinSent(_from, _value, _to);
                        _success = true;
                        _success = true;
                    }
                    else _success = false;
                }

                function coinBalance() constant returns (uint _r) {
                    _r = accounts[msg.sender].balance;
                }

                function coinBalanceOf(address _addr) constant returns (uint _r) {
                    _r = accounts[_addr].balance;
                }

                function approve(address _addr) {
                    accounts[msg.sender].withdrawers[_addr] = 340282366920938463463374607431768211456;
                }

                function isApproved(address _proxy) returns (bool _r) {
                    _r = (accounts[msg.sender].withdrawers[_proxy] > 0);
                }

                function approveOnce(address _addr, uint256 _maxValue) {
                    accounts[msg.sender].withdrawers[_addr] += _maxValue;
                }

                function isApprovedOnceFor(address _target, address _proxy) returns (uint256 _r) {
                    _r = accounts[_target].withdrawers[_proxy];
                }

                function disapprove(address _addr) {
                    accounts[msg.sender].withdrawers[_addr] = 0;
                }
            }

My contract abi :

    var currencyContract = web3.eth.contract([{"constant":false,"inputs":[{"name":"_addr","type":"address"}],"name":"disapprove","outputs":[],"type":"function"},{"constant":false,"inputs":[{"name":"_target","type":"address"},{"name":"_proxy","type":"address"}],"name":"isApprovedOnceFor","outputs":[{"name":"_r","type":"uint256"}],"type":"function"},{"constant":false,"inputs":[{"name":"_proxy","type":"address"}],"name":"isApproved","outputs":[{"name":"_r","type":"bool"}],"type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_value","type":"uint256"},{"name":"_to","type":"address"}],"name":"sendCoinFrom","outputs":[{"name":"_success","type":"bool"}],"type":"function"},{"constant":false,"inputs":[{"name":"_addr","type":"address"},{"name":"_maxValue","type":"uint256"}],"name":"approveOnce","outputs":[],"type":"function"},{"constant":true,"inputs":[{"name":"_addr","type":"address"}],"name":"coinBalanceOf","outputs":[{"name":"_r","type":"uint256"}],"type":"function"},{"constant":false,"inputs":[{"name":"_value","type":"uint256"},{"name":"_to","type":"address"}],"name":"sendCoin","outputs":[{"name":"_success","type":"bool"}],"type":"function"},{"constant":true,"inputs":[],"name":"coinBalance","outputs":[{"name":"_r","type":"uint256"}],"type":"function"},{"constant":false,"inputs":[{"name":"_addr","type":"address"}],"name":"approve","outputs":[],"type":"function"},{"inputs":[],"type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"name":"from","type":"address"},{"indexed":false,"name":"value","type":"uint256"},{"indexed":true,"name":"to","type":"address"}],"name":"CoinSent","type":"event"}]);

then

var currency = currencyContract.at(["0x956440edc6115aab375ab1b96603b79376484d86"]);


And whenever i call a method i get the error.(currency.coinBalance() )

Go version go version go1.5.3 darwin/amd64