error if no value in textbox1 or textbox2

Hi

I get an error when commandbutton1 is selected if there is no value in either textbox1 and textbox2. If there is no value in textbox1 or textbox2 can there be a messagebox pop up saying value must be entered otherwise commandbutton will not work?

Private Sub CommandButton1_Click()
TextBox3.Value = WorksheetFunction.RoundUp(WorksheetFunction.Pi * (TextBox1.Value / 2) ^ 2 * TextBox2.Value / 1000000, 0)
End Sub

Thanks