Excel-VBA: fist and last row calculation wrong - excel

Question

this is my first VBA project and I'm stuck on one problem:
I need to calculate the monthly skewness using daily returns for multiple assets. First, I detected the months and the cells where each month ends and put them in a new worksheet. Based on that, I calculate the monthly skewness for each asset to get a table with the month in the rows and the assets in the columns.
The problem is that the skewness for the first and last row (so the first and last month) are incorrect, although the month is correctly identified by the first part of the code. In the second part of the code where I calculate the skewness I get this error: "Unable to get the Skew property of the WorksheetFunction class" for the line where I want to store the skew in an array but the skewness is calculated nonetheless. How can I solve the error message and get the correct skewness for the first and last month?
Any help would be very appreciated!
This is the code im working with:
Sub step1()
Dim v()
Dim idm()
Dim MonthCount As Single
'find the last row
lastrow = Worksheets("Data").Range("A2").End(xlDown).Row
'save the dates to array
v = Worksheets("Data").Range("A2:A" & lastrow).Value
'count the number of months
M = 1 'initiate month counter
For Row = 1 To lastrow - 2
'extract months from dates
month1 = month(v(Row, 1))
month2 = month(v(Row + 1, 1))
'increase the counter when the month changes
If month1 <> month2 Then
M = M + 1
End If
Next
MonthCount = M
'resize the month arrays
ReDim idm(1 To MonthCount, 1 To 2)
'detect change in month and save cumulative days as row indexes
M = 1 'initiate month counter
For Row = 1 To lastrow - 2
month1 = month(v(Row, 1))
month2 = month(v(Row + 1, 1))
If month1 <> month2 Then
'save the month
idm(M, 1) = month1 & "/" & Year(v(Row, 1))
'save the cumulative days
idm(M, 2) = Row + 1
M = M + 1
End If
On Error Resume Next
Next
'save the last month, cannot detect month change with empty row below
idm(M, 1) = month1 & "-" & Year(v(Row, 1))
idm(M, 2) = Row
'End With
'write the month array to excel
Worksheets("Months").Range("A2:B" & MonthCount + 1).Value = idm
End Sub
Sub step2()
Dim idMonth()
'save last month index to array
lastmonth = Worksheets("Months").Range("B2").End(xlDown).Row
idMonth() = Worksheets("Months").Range("B2:B" & lastmonth).Value
Dim z As Variant
Dim r1, r2 As Double
nc = Worksheets("Data").Range("A1").CurrentRegion.Columns.Count
'initiate counters
r1 = 2
r2 = 2
For c = 1 To nc
For M = 1 To UBound(idMonth) 'm is month index
'end of the month
r2 = idMonth(M, 1)
'set range for skewness
Set month_range = Worksheets("Data").Range(Cells(r1, c + 1).Address, Cells(r2, c + 1).Address)
'store skew in array
'error in the following line
z = Application.WorksheetFunction.skew(month_range)
'write skew to cells
Worksheets("Months").Cells(M + 1, c + 2).Value = z
'start of next month
r1 = r2 + 1
On Error Resume Next
Next
Next
End Sub