为什么将参数传递给函数时出现编译器错误？

Question

我的excel vba工作簿中有以下代码：

Option Explicit
Option Base 1

Private Columns() As Column

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)


    If (Not isInitialised(Columns())) Then
        Call Setup
        Debug.Print "Setup called"
    End If


    '...do the rest

End Sub

Function isInitialised(a() As Variant) As Boolean
    isInitialised = False
    On Error Resume Next
    isInitialised = IsNumeric(UBound(a))
End Function

当Workbook_SheetChange子触发器触发时，出现以下错误：

Compile error: Type mismatch: array or user-defined type expected

将Columns数组传递给isInitialised函数的正确方法是什么？