怎么将null做为参数传递给自定义函数

如何将null做为参数传递给自定义函数
Public Function yesToOne(stra As String)
  If IsNull(stra) Then
  yesToOne = 0
  Else
  If stra = "是" Then
  yesToOne = 1
  Else
  yesToOne = 0
  End If
  End If
End Function

如果传入的参数为空,就会出错.这个怎么改?谢谢.

[解决办法]
Public Function yesToOne(stra As variant)
If IsNull(stra) Then
yesToOne = 0
Else
If stra = "是" Then
yesToOne = 1
Else
yesToOne = 0
End If
End If
End Function