# Gmx trjcat -demux command causes fatal error: "Sorry, write\_trxframe can not write unknown"

**URL:** <https://gromacs.bioexcel.eu/t/gmx-trjcat-demux-command-causes-fatal-error-sorry-write-trxframe-can-not-write-unknown/761>\
**Category:** User discussions\
**Created:** [September 3, 2020, 12:51pm UTC](https://gromacs.bioexcel.eu/t/gmx-trjcat-demux-command-causes-fatal-error-sorry-write-trxframe-can-not-write-unknown/761 "2020-09-03T12:51:10Z")\
**Posts on this page:** 1\
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**Author:** ![winterdl](https://avatars.discourse-cdn.com/v4/letter/w/a8b319/32.png) [@winterdl](https://gromacs.bioexcel.eu/u/winterdl)\
**Post date:** [September 4, 2020, 7:23am UTC](https://gromacs.bioexcel.eu/t/gmx-trjcat-demux-command-causes-fatal-error-sorry-write-trxframe-can-not-write-unknown/761/2 "2020-09-04T07:23:34Z")

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Seems like I am not the first to encounter this issue: [Problem demuxing a PTReplica Exchange](https://gromacs.bioexcel.eu/t/problem-demuxing-a-ptreplica-exchange/502)

But back then no one had any suggestion either. '^\_^

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