问访问标准表达式中的交叉表数据类型不匹配

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enter code here
'assumes that the rate is monthly
Public Function payFrequency(frequency As String, rate As Currency) As Currency
If frequency = "BI-WEEKLY" Then
    payFrequency = rate * 12 / 26
    Exit Function
ElseIf frequency = "SEMI-MONTHLY" Then
    payFrequency = rate * 12 / 24
    Exit Function
ElseIf frequency = "MONTHLY" Then
    payFrequency = rate
    Exit Function
Else
    payFrequency = 0
    Exit Function
End If
End Function

TRANSFORM 
  payfrequency([structurepayrollFrequency],
  Max([2015_PREMIUM]-[2015_EMPLOYER_CONTR])) AS 2015_EMPLOYEE_CONTR