Its showing a syntax error "yield is not declared. it may be inaccessible due to its protection level"

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VB

this is my code which I am going to convert it to vb.net. Please help me out

public IEnumerable<DateTime> GetAllQuarters(DateTime current, DateTime past)
   {
       var curQ = (int)Math.Ceiling(current.Month / 3.0M);
       var lastQEndDate = new DateTime(current.Year, curQ * 3, 1).AddMonths(-2).AddDays(-1);

       do
       {
           yield return lastQEndDate;
           lastQEndDate = lastQEndDate.AddMonths(-3);
           lastQEndDate = new DateTime(lastQEndDate.Year, lastQEndDate.Month, DateTime.DaysInMonth(lastQEndDate.Year, lastQEndDate.Month));
       } while (lastQEndDate >= past);
   }

What I have tried:

Public Function GetAllQuarters(current As DateTime, past As DateTime) As IEnumerable(Of DateTime)
Dim curQ = CInt(Math.Ceiling(current.Month / 3D))
Dim lastQEndDate = New DateTime(current.Year, curQ * 3, 1).AddMonths(-2).AddDays(-1)

Do
yield Return lastQEndDate
lastQEndDate = lastQEndDate.AddMonths(-3)
lastQEndDate = New DateTime(lastQEndDate.Year, lastQEndDate.Month, DateTime.DaysInMonth(lastQEndDate.Year, lastQEndDate.Month))
Loop While lastQEndDate >= past
End Function