runtime error with NSDecimalNumbers?

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C

Objective-C

I am getting an error when this line of code runs, I am unfamiliar with NSDecimalNumber so Im not really sure what is wrong with this line and Xcode is not giving me any error until I actually run the code.

if ([unit1 isEqualToString: [_Distance objectAtIndex:0]])
        universalAns = [num1 decimalNumberByMultiplyingBy: (NSDecimalNumber*)[NSDecimalNumber numberWithDouble:0.0254]];

here is the debugger output up to this point:

self (ViewController *) 0x8c8cd10
unit1 = (__NSCFConstantString *) @"inches"
universalAns NSDecimalNumber * nil
num1 (__NSCFNumber *) (double)0
_Distance (__NSArrayI *) @"8 objects"

Any ideas would be helpful,
Thanks in advance!