[NTG-context] Running standalone metapost file omits spaces between btex and etex

I found that the following example \starttext \startuseMPgraphic{test} draw btex some text with space etex; \stopuseMPgraphic \useMPgraphic{test} \stoptext can generate the text between "btex" and "etex" correct no matter I use MKII or MKIV. But, when I running the following example beginfig(0); draw btex some text with space etex; endfig; end with the command line: $ mpost --mem=metafun test.mp or $ texexec --mptex test.mp It dose generate output. But the output omits all the spaces between btex and etex, with the resulting string to be "sometextwithspace". So, the question is how to get the correct results when running standalong metapost file with btex-etex strings. Thanks.