Why do I get InvalidCastException when casting a double to decimal

A suggestion: try using Convert.ToDecimal() instead of direct casting.

Eric Lippert has blogged about exactly this in depth. I agree it's unintuitive at first, but he explains it well: Representation and Identity

You need to cast it to a double first as row[denominator] is a double boxed as an object i.e.

decimal d = (decimal)((double)row[denominator]);