MDX statement error

Hi Experts

I intend to write a member formula simply being (A+B)/C

So in the planning and admin console, I have written a member formula as follows;

[A]+[B]/[C]

Where [A] and [B] are parent members

[C] is a base member

I can process the dimension successfully, however when I add the specific member into an input form in the excel interface, I was prompted with an "MDX statement error: "value xxxxxxx/CPMB/XWDZYD3 for characteristic /"

I then edit my formula into

[ACCOUNT].[A]+[ACCOUNT].[B]/[ACCOUNT].[C]

dimension processed successfully, but still prompted with the same error.

Please give some advice! as member formula function plays a role in the current model we are trying to build!

your advice is much appreciated

Best Regards