I have a worksheet that contains sample results for chemical analysis.

Not all analytes are analyzed each time, thus some of the cells are empty.

To make the graphs more usable, I want to search the data range for empty
cells and place =NA() in them so that the trend lines and continuous.

Here's what I have been trying...
For Each cl In Worksheets("Data").Range("A2:AC" & LastRowUsed).Cells
If cl.Value = "" Then
cl.Value = "=NA()"
End If
Next

Each time I have tried this, the code errors out on me when it reaches a
cell that already has the value set to =NA().

Is there a better approach to what I'm attempting, perhaps a way to set a
command to do the entire range at once, instead of evaluating each cell?

Thanks in advance, and HAPPY HALLOWEEN.