Textbox Error When Changing Number

This code works fine, the problem is if I put a number in, and then backspace in the textbox to change the number, I get a runtime 13, type mismatch. This error only occurs when I delete all the numbers in the cell. For example, say the number in the cell is 5025. If i change it to 5123, that is I did not have to delete the first 5, it will work. If I want to change to something like 4167, i must delete the first 5 and it will give the error. I have only been using VBA for a couple weeks, I assume it's something simple.

Private Sub TextBox1_Change()
Dim StaffCost As Single
Dim AvgDriveTime As Single
Dim AddlDrives As Single
StaffCost = OLEObjects("TextBox1").Object.Value
AvgDriveTime = Range("E5").Value
AddlDrives = Range("F5").Value
With Range("G5")
.Font.Bold = False
.Value = StaffCost * AvgDriveTime * AddlDrives
End With
End Sub