New Post: Referring a formula in another worksheet

In C#, I am using

cell.SetFormulaA1("SUM('sheet1'!C8:'sheet1'!J8) + SUM('sheet2'!C9:'sheet2'!J9)");

to set a formula that uses other worksheets.

But then when I do this:

string content;
if (!cell.TryGetValue(out content)
{
...
}

it throws an exception.

What am I doing wrong?