LINEST Type Mismatch Error

I have built a user defined function called: portAlpha.

Code:
Function portAlpha(assetCode As String, indexCode As String) As Double

Dim assetCol As Long
Dim indexCol As Long
Dim assetData As Range
Dim indexData As Range

With Sheets("Data")

[Code] ......

I am struggling to get this to work though. Everytime I step over this piece of code:

Code:
portAlpha = Application.WorksheetFunction.LinEst(Range("M4:M63"), Range("N4:N63"), 1, 1)

I get a "Type Mismatch Error". All the values in the range are numbers, and LINEST should return a double, so I am puzzled as to why there is a mismatch...

I have hardcoded the range into the formula just to try and get it to work. Eventually this will be two variables and will look like this:

Code:
portAlpha = Application.WorksheetFunction.Index(Application.WorksheetFunction.LinEst(assetData, indexData, 1, 1),1, 1)