CRAMFS on arm : how to mount with busybox?

I create cramfs, I mount on pc and works
I mount with busybox on target and it doesn't work.
Why?
Thank you and bye bye

mkcramfs skin skin.cramfs
sudo mount -o loop -t cramfs skin.cramfs cramfs-mount/
dd if=skin.cramfs of=/dev/mtd7


bash# busybox mount -t cramfs /dev/mtd7 cramfs-mount/
mount: Mounting /dev/mtd7 on /cramfs-mount failed: Block device
required
bash# strace /busybox mount -t cramfs /dev/mtd7 cramfs-mount/
execve("/busybox", ["/busybox", "mount", "-t", "cramfs", "/dev/mtd7",
"cramfs-mount/"], [/* 10 vars */]) = 0
brk(0)                                  = 0x202af58
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or
directory)
open("/lib/v5l/libc.so.6", O_RDONLY)    = -1 ENOENT (No such file or
directory)
stat("/lib/v5l", 0xbffff840)            = -1 ENOENT (No such file or
directory)
open("/lib/libc.so.6", O_RDONLY)        = 4
fstat(4, {st_mode=S_IFREG|0755, st_size97%4744, ...}) = 0
read(4, "7ELFa(x20"...,
4096) = 4096
old_mmap(NULL, 965788, PROT_READ|PROT_EXEC, MAP_PRIVATE, 4, 0) =
0x4001e000
mprotect(0x400fa000, 64668, PROT_NONE)  = 0
old_mmap(0x400fe000, 36864, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED, 4, 0xd8000) = 0x400fe000
old_mmap(0x40107000, 11420, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x40107000
close(4)                                = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x40015000
personality(PER_LINUX)                  = 8388608
getpid()                                = 28
brk(0)                                  = 0x202af58
brk(0x202bf70)                          = 0x202bf70
brk(0x202c000)                          = 0x202c000
brk(0x202e000)                          = 0x202e000
stat("/dev/mtd7", {st_mode=S_IFCHR|0644, st_rdev=makedev(90, 14), ...})
= 0
getcwd("/", 4097)                       = 2
mount("/dev/mtd7", "/cramfs-mount", "cramfs", 0xc0ed0000, 0x202cf80) =
-1 ENOTBLK (Block device required)
write(2, "mount: ", 7mount: )                  = 7
write(2, "Mounting /dev/mtd7 on /cramfs-mo"..., 42Mounting /dev/mtd7 on
/cramfs-mount failed) = 42
write(2, ": Block device required\n", 24: Block device required
) = 24
_exit(1)                                = ?