When i use the following code with interop.excel

rng = wsDiagram.get_Range("B4", missing);


rng.Formula = "=AVERAGE(B1:B3)"; // OR "=SUM(B1:B3)";

rng.Calculate();

The Cell shows #name?
When i hit the fomula bar suddenly te #name? goes away and the formula works.
Very strange, it happens with all the formulas in de Worksheet :(
And the follwing code works fine filling the cells


rng = wsDiagram.get_Range("B4", missing);

rng.Formula = "=B1";

//OR

rng.Value2 = 43443512;


Can someone help me?
According to MSDN i'm doing it good, but it doesnt work.
PS: i dont get any error messages