This can be a widespread drawback in Fortran. Strategies

put in an if assertion earlier than that decision and cease the execution and print out the zero worth, or debug on the print assertion and look to see the supply. 

work backwards from the road in search of the error that offers you a zero worth, it’s tedious but it surely typically works., utilizing print statements in search of the supply of the zero except it’s apparent  I would depart the if assertion in after you repair it so it occurs from one other supply it ought to no less than print one thing.