Data type mismatch in criteria expression

I am trying to write VBScript in an external application that updates a record in an Access database using an SQL update query. I am using the following:

Function RunSQL(sql_cmd, database)
	Dim rs

	If ENABLE_SQL_LOGGING = 1 Then
        Dim myfile
        Set myfile = file.Open(SQL_LOG_FILE, openReadWrite) ' open file to append
        myfile.Write strEmpName & " - " & Now & " - " & sql_cmd & vbCrLf              ' write data
        myfile.Close                               ' close file
        Set myfile = Nothing                       ' delete object
    End If
	
    If database = "SQL" Then
		Set rs = CreateObject("ADODB.Recordset")
		rs.Open sql_cmd, dbSQLConnection, 2, 3
		Set RunSQL = rs
	ElseIf database = "Access" Then
		Set rs = CreateObject("ADODB.Recordset")
		rs.Open sql_cmd, dbAccessConnection, 2, 3
		Set RunSQL = rs
	End If
	
End Function

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When I run the following SQL I get the error 'Data type mismatch in criteria expression'.

Set rs = RunSQL ("UPDATE LabourTimes SET LabourTimes.StopDate = '" & StartTime & "' WHERE LabourTimes.EmployeeID = '" & EmpID & "' AND IsNull(LabourTimes.StopDate);", "Access")

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The LabourTimes.Employee field is a Long Integer and EmpID is set to 002001. To be sure, I have tried CLng(EmpID) but still get the same error.

If I remove LabourTimes.EmployeeID = '" & EmpID & "' AND from the criteria it works fine, so I am sure it is something to do with this.

I use the following elswhere in the script and it works fine.

strSql = "INSERT INTO LabourTimes (EmployeeID, WIPJob, Operation, WorkCentre, StartDate, ChargeRate) VALUES('" & EmpID & "','" & WIPJob & "','" & Operation & "','" & WorkCentre & "','" & StartTime & "','" & ChargeRate & "')"

Set rs = RunSQL(strSql, "Access")  

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I can not work out what the problem is. Please help.