Oraclize: oraclize_newRandomDSQuery always reverted

Question

I'm using oraclize for generating random number for my smart contract. I have tested successfully with 2 methods:

• Using WolframAlpha
• Using random.org service

But when I try oraclize_newRandomDSQuery. My transaction always reverted. I don't know why Here is my code:

function _sendRandomQuery(uint256 _gasLimit) internal returns (uint256) {
           oraclize_newRandomDSQuery(0, 7, 200000);
           return 100;

}

Please tell me why.