Empty Textbox = Program crashes?

The program I have written keeps crashing if the textbox is not filled in..how do I Prevent this?

this is the code I have this far..
'to make sure that user enters week number less than 52

If (txtReportingPeriod.Text > 52) Then
MessageBox.Show("Please enter the week number for this payroll period. Number may not be over 52.", "Error-Incorrect week number submitted.")

End If