Issue calling contract

gistfile1.js

> var contract_address = '0x5a219cab7d5a5888ae86e80de2775dc022de6e0c';
undefined
> abiDef = [{ type: 'function', constant: true, inputs: [ ],  name: 'step', outputs: [{name: 'retVal', type: 'uint256'    } ]  }]
[{
  type: 'function',
  constant: true,
  inputs: [ ],
  name: 'step',
  outputs: [{
    name: 'retVal',
    type: 'uint256'
  } ]
} ]
> counter_contract = eth.contract(abiDef);
{
  abi: [{
    type: 'function',
    constant: true,
    inputs: [ ],
    name: 'step',
    outputs: [{
      name: 'retVal',
      type: 'uint256'
    } ]
  } ],
  new: [Function],
  at: [Function]
}
> counter = new counter_contract(contract_address)
TypeError: 'counter_contract' is not a function
    at <anonymous>:1:15

counter = counter_contract.at(contract_address)