An EA that uses several timeframes

I have created an EA with several timeframes, however, when tested, bugs are encountered, please advise.

I am using a H4 timeframe when going through a test.

input int      barNumM=42;             // Number of bars in mid term array
double   SLS=0;


MqlRates mrateS[];
MqlRates mrateM[];

if(CopyRates(_Symbol,PERIOD_H4,0,barNumS,mrateS)<0)
{
Alert("Error copying H4 rates/history data - error:",GetLastError(),"!");
return;
}
if(CopyRates(_Symbol,PERIOD_D1,0,barNumM,mrateM)<0)
{
Alert("Error copying D1 rates/history data - error:",GetLastError(),"!");
return;
}

SLS=mrateM[iHighest(_Symbol,PERIOD_D1,MODE_HIGH,barNumM,0)].high-mrateM[iLowest(_Symbol,PERIOD_D1,MODE_LOW,barNumM,0)].low;

Bug: mrateM[iHighest(_Symbol,PERIOD_D1,MODE_HIGH,barNumM,0)].high-mrateM[iLowest(_Symbol,PERIOD_D1,MODE_LOW,barNumM,0)].low returns negative values sometimes.

Some of my buy logic is not working properly as well, but I think that is related to mrateM not functioning properly.