libvlc crashes when trying to play HEVC mkv files

When i try to play a HEVC mkv file in my custom player (written in c# using libvlc and libvlcsharp) it crashes out. Just for background my player works, and has worked for many months, just not with this type of file. When I attach windbg I get the following:-

(5d30.5558): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
libvlccore!vlc_mutex_lock+0x8:
00007ff8059fe9c8 803900          cmp     byte ptr [rcx],0 ds:0dd019412b5a094c=??

with a stack trace of:

[0x0]   libvlccore!vlc_mutex_lock + 0x8
[0x1]   libvlccore!vlc_obj_free + 0x85c
[0x2]   libvlccore!var_DelCallback + 0x1b
[0x3]   liblibass_plugin + 0x2785
[0x4]   libvlccore!vlc_ureduce + 0x1a6
[0x5]   libvlccore!input_Control + 0x264c
[0x6]   libvlccore!vlc_demux_chained_Delete + 0x3713
[0x7]   libvlccore!vlc_demux_chained_Delete + 0x60fe
[0x8]   libvlccore!vlc_demux_chained_Delete + 0x703
[0x9]   libvlccore!vlc_demux_chained_Delete + 0x6887
[0xa]   libvlccore!vlc_demux_chained_Delete + 0xdd73
[0xb]   libvlccore!vlc_demux_chained_Delete + 0x121e8
[0xc]   libvlccore!input_GetItem + 0x90e
[0xd]   libvlccore!input_Read + 0x153
[0xe]   libvlccore!vlc_rand_bytes + 0x986
[0xf]   msvcrt!_callthreadstartex + 0x1e
[0x10]   msvcrt!_threadstartex + 0x7c
[0x11]   KERNEL32!BaseThreadInitThunk + 0x14
[0x12]   ntdll!RtlUserThreadStart + 0x21

Any help would be appreciated

thanks

Steve