Set a breakpoint at that line in your function, and when the breakpoint is triggered, check the class of D. I suspect you'll find it is a cell array.
The problem may be that you have some other variable in the function, also called D, which is a cell array, and which is defined later than the double D, and so it is replacing the initially-defined double D. (This is why it's necessary to check the variable(s) when the line is actually executed.)
