Accessing elements in an mxArray that contains only numbers

I am using the MATLAB engine and xlsread to read an excel spreadsheet. I use xlsread in the following way:

engEvalString(ep, "[num, text, raw] = xlsread('C:\\rest_of_file_path', 'Data'");

However when I try to use mxGetCell(num,1) or any other index (i.e. mexGetCell(num, 0), mxGetCell(num, 30), mxGetCell(num,4)) it throws the following exception:

Access violation reading location 0x3f7a9fc7.

But when I use either text or raw instead of num it doesn't throw an exception. For example, the following two statements throw no exceptions:

mxGetCell(raw,1)

mxGetCell(text,1)

Does anyone know why this could be happening?

Thanks!

[James Tursa]

Is num NULL? How are you importing num, text, and raw into your program?

[Caroline]

I am using engGetVariable to get them into my C++ program