Simple VBA Degub help

I am learning VBA using the dummies book, but need this to work now and would appreciate any help, it seems like it should be a simple answer.

Private Sub ComboBox20_Change()
If ComboBox20.Value = "Other (Specify)" Then TextBox18.Visible = True
If ComboBox20.Value <> "Other (Specify)" Then TextBox18.Visible = False

End Sub

This code is just to make a text box appear or disappear depending on what is selected in a combo fox. SOMETIMES (but not always) when people open this spreadsheet i get 4 warnings for the 4 appearances of this code, with the opportunity to debug and it highlights the line:

If ComboBox20.Value <> "Other (Specify)" Then TextBox18.Visible = False

My theory is that this error's appearance is connected to whether the text boxes have been manually adjusted to true/false visibility in the properties box but im not sure. Could be missing something simple!

Thanks for any help.

Sincerely,

Alex