I'm trying to pass a string argument value to an MPgraphic but it doesn't seem to work:

\startuseMPgraphic{name}{lbl}
pair a ; a := (3cm,3cm) ;label.top("\MPvariable{lbl}",a) ;
\stopuseMPgraphic

\starttext
\useMPgraphic{name}{lbl=1}
\stoptext

The above works as expected!

But the following doesn't:

\startuseMPgraphic{name}{lbl}
pair a ; a := (3cm,3cm) ;label.top("\MPvariable{lbl}",a) ;
\stopuseMPgraphic

\starttext
\useMPgraphic{name}{lbl=abc}
\stoptext

1     \startuseMPgraphic{name}{lbl}
2       pair a ; a := (3cm,3cm) ;label.top("\MPvariable{lbl}",a) ;
3     \stopuseMPgraphic
4
5     %\startuseMPgraphic{name}fill fullcircle scaled 200pt withcolor .625yellow ;\stopuseMPgraphic
6
7     \starttext
8      Example:
9 >>    \useMPgraphic{name}{lbl=abc}
10     \stoptext
11

<to be read again>
a
\m_meta_current_variable a-a
                            bc
<argument> ... \hbox {\scratchdimen \m_meta_current_variable
                                                  sp}\ifzeropt \wd \b_meta_v...
\secondoftwoarguments #1#2b-#2

\9>>1 #1,2-\meta_prepare_instance_variable {#1}
                                               \syst_helpers_do_process_comm...
\syst_helpers_do_process_comma_list ...rgument #1,
                                                  ]\relax \global \advance \...
...
l.9   \useMPgraphic{name}{lbl=abc}