Type mismatch error obtaining value from a cell

Hi there,

This is probably quite simple, but I haven't found the answer on the forum or elsewhere... please help

Dim ProjectNumber As Long
Dim TableRow As Integer
TableRow = 6

Do While Not IsEmpty(Workbooks(BacklogWorkbook).Sheets("Backlog Data").Range("a" & TableRow))

    ProjectNumber = Workbooks(BacklogWorkbook).Sheets("Backlog Data").Range("d" & TableRow)

' rest of the Do While loop follows

So the idea is that it is going through a set of data one row at a time (using TableRow), and the first task is to extract the ProjectNumber from column d of the row.

The final line of my code (ProjectNumber = etc) gives me a type mismatch error when I run it.
The same result occurs with:

Dim ProjectNumber as Integer

On the other hand, if I do this:

Dim ProjectNumber as Range

I get this error instead:
Object variable or With block variable not set

...which makes no sense to me as ProjectNumber is not an object (unless Range is an object of course, but in that case surely I have set it?)

Confused!!

Any help appreciated... thanks

ssu95bm