Run-time error 13 (Type Mismatch)

hi,
i am a new user of module in excel.

When i run the following code i get the "Type mismatch" error dialogue box. It was running fine a few hours ago.

What is amusing is that it sometimes give the "Application or object-defined" error dialogue box as well.

What shall i do to resolve this? And, what could be causing 2 different error dialogue boxes for the same code?

Sub addvalues()

For i = 2 To Worksheets.Count

    For j = 12 To 87


        For k = 6 To 20
    
             Worksheets(1).cells(j, k) = Worksheets(1).cells(j, k) + Worksheets(i).cells(j, k)
    
         Next k
    
    Next j

Next i

End Sub