Not getting error message I need, instead getting Out of Range Error (Error 9)

Hello everyone! I'm new to the forum, but have been using it for many of my projects over the past few months. In the company i work for we cannot get Microsoft Access so I am basically circumventing this issue and creating our own database using multiple excel files. There are about 12 files talking to each other, and so far the only issue i have run into was the below. Since i can't search tab names easily, I just am opening the database, and then the tab based on user input which will be a machine's serial number. The database will have a separate sheet for each machine that is QC tested, and where I've run into an issue is in the error. Instead of returning the error message I want, I get "Subscript out of range (Error 9)".

Sub Database_Search()

If Range("B9").Value = "SXD2010A-M" Then
    userinput = InputBox("Enter Serial Number: ")
    'A20 is set as a text field
    Range("A20").Value = userinput
    Set A = Range("A20")
    'Open database
    Workbooks.Open "C:\Documents and Settings\...etc"
    'Open up sheet based on input field
    Sheets(A.Value).Activate
    'On error show msg box "Not in Database"
    On Error GoTo ErrorHandler
    Exit Sub
    
    Else
    End If

ErrorHandler:
    MsgBox ("Not in Database")

End Sub