Function Works In VBA But Does Not Work Inside Worksheets

i have written a code in VBA to interpolate the value of Y0 corresponding to X0 using a set of (X,Y) points. (I have written this in module1) . This function works when i call it through a Sub or another Function in VBA. but it does not work when I try to use it as a function in my excel worksheets (when I type : =interpolate(A1:A10,B1:B10,30) .... 30 is an arbitrary value). in this case I get #value! error

The function is known in within the worksheets because when I start typing its name, the Auto Name Complete feature of excel, finds this function.

I am new to VAB for excel. Please give me a hint to see my mistakes of if something is missing inside my code.

PHP

Public Function Interpolate(ByRef X() As Double, ByRef Y() As Double, ByRef X0 As Double) As Double    Dim I As Integer, Slope As Double, NData As Integer        NData = UBound(X)    For I = 1 To UBound(X) - 1        If (X(I) = X0) Then            Interpolate = Y(I)            Exit Function        ElseIf (X0 < ListMax(X(I), X(I + 1)) And X0 > ListMin(X(I), X(I + 1))) Then            Slope = (Y(I) - Y(I + 1)) / (X(I) - X(I + 1))            Interpolate = Y(I + 1) + Slope * (X0 - X(I + 1))            Exit Function        End If    Next I        End FunctionPublic Function ListMax(ParamArray ListItems() As Variant)        Dim I As Integer        ListMax = ListItems(0)        For I = 0 To UBound(ListItems())            If ListItems(I) > ListMax Then ListMax = ListItems(I)        Next IEnd FunctionPublic Function ListMin(ParamArray ListItems() As Variant)        Dim I As Integer        ListMin = ListItems(0)        For I = 0 To UBound(ListItems())            If ListItems(I) < ListMin Then ListMin = ListItems(I)        Next IEnd Function