diagram gets messed up by adding text

[Reissner]

I have an mp file as follows:

input metauml;
%defaultscale := 0.8;
%defaultscale := 10pt/fontsize defaultscale;
beginfig(1);
%defaultscale := 8pt/fontsize defaultscale;
%defaultscale := 0.8;

Actor.devel("developer");
Class.editor("source editor")()();
Class.sviewer("source viewer")()();
Class.tviewer("target viewer")()();
Class.lviewer("log viewer")()();
Class.console("console")()();
devel.n + (-100, 50) = editor.sw;
devel.n + ( +50, 50) = sviewer.s;
devel.s - (   0, 50) = console.n;
devel.w - (  50,  0) = lviewer.e;
item(iAssoc)("typing")(obj.sw = 0.5[devel.n,editor.se]);
%item(iAssoc)("command")(obj.se = 0.5[devel.s,console.n]);

%label.top("test", (100,0));
drawObjects(devel, editor, sviewer, lviewer, console);%, tviewer, console

link(inheritance)(devel.n -- editor.se);
link(inheritance)(editor.sw -- devel.n);

link(realization)(sviewer.s -- devel.n);
link(realization)(lviewer.e -- devel.w);

link(realization)(0.7[console.n,console.ne] -- devel.s);
link(inheritance)(devel.s -- 0.7[console.n,console.nw]);


%currentpicture := currentpicture scaled 0.2;
endfig;
end

compilation with

mpost -interaction=nonstopmode -recorder -s prologues=3 -s 'outputtemplate="F3_01texUsagePlain.mps"' F3_01texUsagePlain.mp

works fine.
As you observe, there is a line

%item(iAssoc)("command")(obj.se = 0.5[devel.s,console.n]);

F3_01texUsagePlainMixedUp.pdf

F3_01texUsagePlain.pdf

which is commented out.
If I add this, then compilation yields an error indicating

! Inconsistent equation (off by 97.7471).

and the picture gets messed up.
I converted into PDF what I see without the line and the result with the line above.