dim not working on multiple ranges

I must be doing something basic wrong.

Sub sbTestRanges()
Dim a, b, c As Range
    a = Cells(1, 1)
    b = Cells(1, 1)
    c = Cells(1, 1)
End Sub

I get an error only for c, that the object variable or with block variable is not set.

if I change the dim line to
dim a, b, c, d as range and never use variable d, then the macro runs as I expect with no errors.

Can anyone explain? Visual Basic 6.5 in Excel 2003.