"no top surface node was found" when take surface BC (run with parallel)

Hi all,

I'm taking a 3d box test(P1P1).
When take the surface BC on top it shows warning message below:
Warning: your diagnostic free surface will only be defined at the free surface nodes and
not extrapolated downwards, because you didn't specify geometry/ocean_boundaries.

After specify geometry/ocean_boundaries, it seems can work in single thread.
But when run with parallel (4 thread), beside fluidity.err-0, others get error like below:

For node with coordinate -2.12079668519062695E-002 5.17641830894005686E-002 8.80246248583304962E-003
 no top surface node was found.
*** FLUIDITY ERROR ***
Source location: (Vertical_Extrapolation.F90, 426)
Error message: Something wrong with the geometry.
Backtrace will follow if it is available:
/usr/bin/fluidity(fprint_backtrace_+0x1a) [0x4d274a]
/usr/bin/fluidity(__fldebug_MOD_flabort_pinpoint+0x210) [0x4cb660]
/usr/bin/fluidity(__vertical_extrapolation_module_MOD_horizontal_picker+0xdde) [0x6cb31e]
/usr/bin/fluidity(__vertical_extrapolation_module_MOD_verticalextrapolationmultiple+0x2e9) [0x6cc8d9]
/usr/bin/fluidity(__vertical_extrapolation_module_MOD_verticalextrapolationscalar+0x1b8) [0x6cd6d8]
/usr/bin/fluidity(__vertical_extrapolation_module_MOD_updatedistancefield+0xd1) [0x6cda41]
/usr/bin/fluidity(__vertical_extrapolation_module_MOD_calculatetopbottomdistance+0x1fa) [0x6cdcaa]
/usr/bin/fluidity(__fluids_module_MOD_fluids+0x4ab5) [0x4d2275]
/usr/bin/fluidity(mainfl+0xd3) [0x4cae63]
/usr/bin/fluidity(main+0x112) [0x4c0122]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed) [0x2b7e5e13e78d]
/usr/bin/fluidity() [0x4c14b1]
Use addr2line -e <binary> <address> to decipher.
Error is terminal.

Do i take some option wrong in flml? Any comment is appreciated.

Thanks

Liu