VBA - runtime error 94, invalid use of null

Hi There

I've been using this macro that allows me to select multiple items in a drop down list

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When I run debug it says that there is something to do with the Prior=Target.Text line. I'm a bit out of my depth here - how do I go about debugging this? I've attached my spreadsheet as an example. To get the error to occur, simply continue using the heading that states 'tax concession sought' selecting different items. It sometimes happens straight away, and other times it doesn't happen for a while, but eventually it does...

Register Null Error.xlsm

Thanks in advance

Arneld

It's working perfectly now. Thank you so much for your help!

dude you need to make a selection in c column for it to work

Of course, but regardless of that, the error still occurred. Cheers for your response anyway!