dynamic array causes an error.

I have a macro of which does what I need, however I don't always know what size array could be needed.
I am trying to change my array to dynamic but this causes an error.

My best knowlage to do this is to change

Dim missingDutys(10) As Variant, x As Integer

to

Dim missingDutys() As Variant, x As Integer

BUT DOING THIS FAILS... Why?
My Code

Sub missingDutys()

Dim first_line As Integer, last_line As Integer, rotaTable As Worksheet, mon_Thurs As Worksheet
Dim r1 As Integer, r2 As Integer, found As Boolean

Dim missingDutys(10) As Variant, x As Integer

Set rotaTable = Sheets("Rota Table")
Set mon_Thurs = Sheets("Mon-Thurs Duty Times")

Application.ScreenUpdating = False

    x = 0
    first_line = 7
    last_line = 73
    
' For each entry in the second worksheet, see if it's in the first.
For r2 = 7 To 51
    found = False
    ' See if the r1-th entry on sheet 2 is in the sheet 1 list.
    For r1 = first_line To last_line
        If rotaTable.Cells(r1, 5) = mon_Thurs.Cells(r2, 1) Then
        ' We found a match.
            found = True
            Exit For
        End If
    Next r1

    ' See if we found it.
    If Not found Then
    
        ' Flag this cell.
        ' mon_Thurs.Cells(r2, 1).Interior.ColorIndex = 35
         
        missingDutys(x) = mon_Thurs.Cells(r2, 1)
        x = x + 1
        
        End If
Next r2

    Dim y As Integer
    For y = 0 To UBound(missingDutys())
    rotaTable.Range("T25").Offset(y, 0).Value = missingDutys(y)
    Next y

Erase missingDutys

Application.ScreenUpdating = True


End Sub

Help would be really appreciated.
Thanks.