'Type mismatch' error with autofill and VBA

[The Mekon]

I use the following code to automatically colour a cell if any of the letters shown are typed into it:

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But whenever I use autofill to put the same letter in several cells, the following error is returned (although the cells are coloured!):

Run-time error '13'

Type mismatch

How can I stop this error from showing? Any help is appreciated.

[VBA Noob]

Thank you for wrapping your code.

Try

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VBA Noob

[The Mekon]

It works, thank you!!

Sorry about not wrapping the code earlier.

Thanks for the help