how to stop VBA overwriting Pivot Table Value if Value doesn't exist

Simply put

I have drop down cell for the user to insert the month number 1-12.

I have then writen the following code to set the pivot table.

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The pivot table is reading information off another worksheet which is being populated from the rest of the workbooks.

So as time goes by, each month will get used.

So at present values 1-8 work fine.

However, if the user enters 9, then the current month in the pivot tables get overwritten with the value 9.

What i want is for the code to enter say 9 (presently) and if error then leave the value as is adn msg box "no entries" and stop VBA.

Simples i'm sure, but i can't work it out.

can anyone help?