nc_close causes problems for H5Fopen ?!

I have an application that first writes and closes a NetCDF4 file,
then writes an HDF5 file. If nc_close() is *not* called, writing the
HDF file works fine.

But when nc_close() is called to close the NetCDF4 file, then H5Fopen
throws an exception:

HDF5-DIAG: Error detected in HDF5 (1.8.4-patch1)
thread 0:
   #000: H5F.c line 1510 in H5Fopen(): not file access property list
     major: Invalid arguments to routine
     minor: Inappropriate type
   #001: H5Pint.c line 3032 in H5P_isa_class(): not a property list
     major: Invalid arguments to routine
     minor: Inappropriate type

What could be going on?

Thanks!

  -- Rodney