Combo box listIndex = -1 when entry selected

I have a combo box on form1 that lists employer names in my demographic table. When I select an entry, I store the ListIndex in a table, go to form2. When form2 is closed, the selected value is restored in the combo box on form1 using the ListIndex.  For most selected values this works fine, but for a range of values the ListIndex is set to -1.  For example, This list was generated in SQL Query Analyzer but it is the same list that appears in the combo box. When A-Team is selected the ListIndex = 41, but when A.A. Pesce Glass Co is selected, ListIndex = -1.  ListIndex is -1 for all values selected until the value Athena Bus Service LLC is selected. It has ListIndex = 711. .  Here are the lines of code I'm using when the Replace Employer button is clicked.  The name of the combo box is "employer".  The value is stored in the table correctly.  The question is why are some values assigned ListIndex= -1 and some assigned the correct ListIndex.  The problem occurs whether I replace the value or simply close form2.
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Private Sub replaceEMP_Click()
Dim varLIEmployer As variant
Dim strEmplValue as string
Dim strUserID as string

If IsNull(Me!employer) Or IsEmpty(Me!employer) Then
  MsgBox "Select an employer value"
  Me!employer.SetFocus
  Exit Sub
End If
varLIEmployer = Me!employer.ListIndex
strEmplValue = Me!employer.Value

DoCmd.SetWarnings False
DoCmd.RunSQL "UPDATE SessionData SET indexEmployer = " & varLIFindEmployer & _
             " WHERE userid = '" & strUserid & "'"
DoCmd.SetWarnings True
DoCmd.OpenForm "Find and Replace (REPLACE EMPLOYER)", acNormal, , "employer = '" & g_strEmplValue & "'"

End Sub
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Thanks for your help.