Making an RT-11 boot disk

I'm trying to use simh to create an RT-11 bootable RX50 disk and am following some instructions posted by Megan Gentry a while ago. I've created the disk image but am having trouble making it bootable. The copy/boot command claims it can't find the RT-11 image but it is clearly on the floppy (du0). Any idea what's going wrong? .dir du0: 10-Apr-99 RT11XM.SYS 106P 20-Dec-85 DU .SYS 8P 20-Dec-85 TT .SYS 2P 20-Dec-85 PIP .SAV 30P 20-Dec-85 DUP .SAV 47P 20-Dec-85 DIR .SAV 19P 20-Dec-85 RESORC.SAV 25P 20-Dec-85 EDIT .SAV 19P 20-Dec-85 MACRO .SAV 61P 20-Dec-85 CREF .SAV 6P 20-Dec-85 LINK .SAV 49P 20-Dec-85 LIBR .SAV 24P 20-Dec-85 FILEX .SAV 22P 20-Dec-85 HELP .SAV 132P 20-Dec-85 BATCH .SAV 26P 20-Dec-85 FORMAT.SAV 24P 20-Dec-85 SETUP .SAV 41P 20-Dec-85 SPEED .SAV 4P 20-Dec-85 DATIME.SAV 4P 20-Dec-85 LET .SAV 5P 20-Dec-85 SPLIT .SAV 3P 20-Dec-85 CONFIG.SAV 7P 20-Dec-85 SWAP .SYS 27P 20-Dec-85 23 Files, 691 Blocks 95 Free blocks .copy/boot du0:rt11xm.sys du0: ?DUP-F-File not found DU0:RT11XM.SYS