input numeric value in userform text box

good mornig

in userform macro
there is a textbox box
when I insert positive number working
error with negative value

these are the lines of code

' corquo  is defined  public as double

'-----------------------------------------------------------

Private Sub TextBox_corquo_Change ()   '
corquo = TextBox_corquo.Value               ' work with positive number only

end Sub

'------------------------------------------------------------------------

Private Sub label_fs_Change()
Dim scala As String
fs = Val(label_fs.Text)
End Sub

What do I do accept negative values?

grazie