Streaming operator for unpacking a bitstream

I'm stuck on a problem I encountered with a use of the streaming operator to unpack into a dynamic array.  Rather than write procedural code, I thought I could use it actually with some amount of success.  Here is what the code more or less looks like:

uvm_pack_bitstream_t bit_vector;
bit bit_array[];
...
bit_vector = packer.get_packed_bits();
{<<{bit_array with [0 +: packer.count]}} = bit_vector;
myfunc(bit_array);
 

Yes, I'm trying to use "unadvertised" features of the UVM packer here.  This has worked in many cases, but there is one case that led to the following:

# ** Fatal: Width do not match. Stream larger than variable.