Process only filled out textboxes on cmd button click

[FryGirl]

I have two question relating to a userform.

First, if the user enters values in textbox1 and textbox2, but does not enter anything in textbox4 how can I prevent the error that bascially says textbox4 is empty. Same thing for textbox1 and/or textbox2 are empty yet textbox4 contains a value.

So in the end both textbox1 and textbox2 need to be filled out before they can process a result in Textbox3.

Also, I thought the [hh]:mm would allow the processing of time over 24 hours like it will in spreadsheet cell, but it doesn't seem to be cooperating.

Private Sub CommandButton1_Click()
    TextBox3.Value = Format(TextBox1.Value / TextBox2.Value, "0.00")
    TextBox5.Value = Format(TextBox4 / 1440, "[hh]:mm")
End Sub

 

[FryGirl]

Any thoughts or suggestions?