Crystal报表公式错误字符串是必需的

Question

我正在使用水晶报告，我创建了两个名为startDate和endDate的参数，当我选择公式选择并编写代码并保存时，我收到一条错误消息＆＃34 ;字符串是必需的＆＃34; 代码是：

if {Employees.DateField} >={?startDatePF} and {Employees.DateField}<={?endDate} then true

请告诉我如何解决此错误。

Public Sub GetReport()
  Dim rpt As New CrystalReport1
  rpt.Load(Application.StartupPath & "\CrystalReport1.rpt")
  Dim pfield As New ParameterField
  Dim pfields As New ParameterFields
  Dim pdescrete As New ParameterDiscreteValue
  Dim pfield1 As New ParameterField
  Dim pfields1 As New ParameterFields
End Sub