Excel VBA - IsNumeric

Hi
I am creating a Userform which I need validations on the cells. A number of which are highlighting the cell if a non-numeric character is entered. Having searched the net I have been using this code:

Private Sub EmployeeNumber_KeyPress(ByVal KeyAscii As MSForms.ReturnInteger)

If Not IsNumeric(EmployeeNumber.Text) Then
EmployeeNumber.BackColor = &HFF&
Cancel = True
Else
EmployeeNumber.BackColor = &H80000005
Call EnableApply

End If

End Sub

This works pretty well, however if only a single numeric character is entered, it must take as non-numeric and highlights red.
I would ideally like to have a message box appear as soon as a non-numeric character is entered but due to this problem, it would be very frustrating to the user as the message box would appear as soon as they type in to the cell even if it is numeric.
Could you please explain why it see's a single number as non-numeric and if there is anyway I can change it?
Thank you
Richard