remix里这段编译错误应该怎么修改？

contracts/szairdrop2.sol:23:9:
 Warning: Return value of low-level calls not used. contract_address.call(id, msg.sender, tos[i], vs[i]);

对应源码：

function transfer(address contract_address,address[] tos,uint[] vs)
public
validAddress(contract_address)
returns (bool){
require(tos.length > 0);
require(vs.length > 0);
require(tos.length == vs.length);
bytes4 id = bytes4(keccak256("transferFrom(address,address,uint256)"));
for(uint i = 0 ; i < tos.length; i++){
    contract_address.call(id, msg.sender, tos[i], vs[i]); //这一行报的错
}
return true;
}