If Statement with an InputBox - Not Working

I am trying to write a macro that looks at a cell and tells me if it's empty, and if it is, open up an input box to fill in the cell. The below code is not opening the message box regardless if the cell is empty or not. C37 IS A MERGED CELL which I know could be mucking things up. What am I missing here?

If IsEmpty("C37") Then


Dim SIGN As String          'Sign-off

SIGN = InputBox("Enter Your Name:", "Dispositioned By:")

Range("C37").Value = SIGN
Range("G37").Value = Format(Now(), "MM/DD/YYYY")

Else

Range("B26").Select

End If