Why Does It Not Work?

The on click event, for a Report, has the following:

Sub DateEntry()

Dim Start_Date As Date
Dim End_Date As Date

Start_Date = InputBox("Enter Start Date mm/dd/yyyy")
End_Date = InputBox("Enter End Date mm/dd/yyyy")

End Sub

This works fine and allows for Input. I added the sub below, but it does not work and the report opens:

Sub CheckEntry()

If Start_Date Is Null Then
MsgBox "No Date was Entered"
DoCmd.Close acReport, "Summary Action Report"
Exit Sub
End If


If End_Date Is Null Then
MsgBox "No Date was Entered"
DoCmd.Close acReport, "Summary Action Report"
End If
End Sub