Error message "Scalar structure required for this assignment"

[Michael Bell]

When I run this fragment

.....................................

if Scan(1).Reach <= 300
o = 1;
% Left end marked.
Map.Object(o).LeftMarker = 0;

end
.....................................

I get this error message

....................

??? Scalar structure required for this assignment.

Error in ==> C:\MATLAB6p5\work\Develop03.m
On line 31 ==> Map.Object(o).LeftMarker = 0;

..........................................

And I can't see why. I've assigned numerical values to fields of
structs often enough and no problem. Why this time?

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