VBA Excel Function Error Runtimeerror 13

I cannot find the mistake. The error msg gives me "runtime error '13'" data types don't match. The error seems to be in the function "fillcalweeks".

Fillcalweeks should return an array which is filled with start and end dates which are located in a sheet.

Here's my code:

'For every calenderweek we need the start and end dates in an array to produce the timeline

Sub get_cal_weeks()
    Dim weeks As Integer, i As Integer, col As String, weekstart As Date, weekend As Date
    'start column is D
    col = "D"
    'get amount of weeks
    weeks = countcalweeks()
    'populate array calweeks
    calweeks = fillcalweeks(weeks)
    For i = 0 To weeks

       Sheets("Kalenderwochen").Range("E" & i + 1) = calweeks(i, 1)

    Next
End Sub

Function fillcalweeks(weeks As Integer) As String()
    Dim i As Integer, datestart As Date, dateend As Date, calweek As Integer, returnarray() As String
    For i = 0 To weeks
    'date start & date end
        datestart = Sheets("Kalenderwochen").Range("A" & i + 1).Value
        dateend = Sheets("Kalenderwochen").Range("B" & i + 1).Value
        calweek = Sheets("Kalenderwochen").Range("C" & i + 1).Value
        returnarray(i, 1) = datestart
        returnarray(i, 2) = dateend
        returnarray(i, 3) = calweek
        fillcalweeks = returnarray
    Next
End Function


'Counts the calenderweeks in the Kalenderwochen sheet
Function countcalweeks() As Integer
    countcalweeks = Sheets("Kalenderwochen").Range("A2").End(xlDown).row
End Function

Thx for the help