Data Type Mismatch

I am trying to get this VBA code to work, which allows you to email from Access.
I am getting a 'Data type mismatch in criteria expression'.

I can't see anything that looks wrong to me, but I am not very experienced.
Here is the Code:

Code:

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Private Sub btn_Send_Click()
    On Error GoTo Err_btn_Send_Click

    Dim stWhere As String      '-- Criteria for DLookup
    Dim varTo As Variant        '-- Address for SendObject
    Dim Message As String        '-- E-mail text
    Dim DateSent As Variant      '-- Sent date for e-mail text
    Dim Subject As String    '-- Subject line of e-mail
    Dim EID As String    '-- The ticket ID from form
    Dim stWho As String        '-- Reference to Email_ADDR
    Dim From As String    '-- Person who sent the email
    Dim strSQL As String        '-- Create SQL update statement
    Dim errLoop As Error

    '-- Combo of names to assign ticket to
    stWho = Me.cboTo_Key
    stWhere = "Email_ADDR.ID = " & "'" & stWho & "'"
    '-- Looks up email address from Email_ADDR
    varTo = DLookup("[Email]", "Email_ADDR", stWhere)

    stSubject = "Subject"

    'EID = Format(Me.EID, "00000")

    stText = Me.Message

    'Write the e-mail content for sending to assignee
    DoCmd.SendObject , , acFormatTXT, varTo, , , stSubject, stText, -1

    'Set the update statement to disable command button
    'once e-mail is sent
    strSQL = "UPDATE Email " & _
            "SET Email.Sent = -1 " & _
            "Where Email.EID = " & Me.EID & ";"


    On Error GoTo Err_Execute
    CurrentDb.Execute strSQL, dbFailOnError
    On Error GoTo 0

    'Requery checkbox to show checked
    'after update statement has ran
    'and disable send mail command button
    Me.Sent.Requery
    Me.Sent.SetFocus
    Me.btn_Send.Enabled = False

    Exit Sub

Err_Execute:

    ' Notify user of any errors that result from
    ' executing the query.
    If DBEngine.Errors.Count > 0 Then
        For Each errLoop In DBEngine.Errors
            MsgBox "Error number: " & errLoop.Number & vbCr & _
                  errLoop.Description
        Next errLoop
    End If

    Resume Next


Exit_btn_Send_Click:
    Exit Sub

Err_btn_Send_Click:
    MsgBox Err.Description
    Resume Exit_btn_Send_Click

End Sub

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This is the Email table structure:
EID: Autonumber
Subject: Short Text
Message: Short Text
Date Sent: Date/Time
To Key: Number
Sent: Yes/No

The SQL query looks OK to me, but perhaps I am missing something!

What does strSQL contain when you run it?

is the email sent before the error occurs?

i would believe a string for the where criteria should be enclosed in ' '
like

Code:

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    strSQL = "UPDATE Email " & _
            "SET Email.Sent = -1 " & _
            "Where Email.EID = '" & Me.EID & "';"

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