savecore: Value too large to be stored in data type

has anyone seen this problem on a panic?

i have 4GB of physical memory, running 5.2.1 RC.
I had a panic occur, and it won't write the whole vmcore.

1985347584 is the size of the vmcore i get (not compressed).
Its not useful as far as i can tell, gdb doesn't know where
to begin.

My swap partition is 8GB. The filesystem i'm saving the
vmcore too is plenty big enough.

Mar  4 16:05:09 cdata savecore: write error on vmcore.1 file: Value too
large to
 be stored in data type
Mar  4 16:05:09 cdata savecore: WARNING: vmcore may be incomplete
Mar  4 16:05:09 cdata savecore: unsaved dumps found but not saved

I'm not sure what 'value to large...' message means in this context.
Can someone suggest how to get my core file?

$ swapinfo 
Device          1K-blocks     Used    Avail Capacity
/dev/da0s1b       8388608        0  8388608     0%

dumpdev="/dev/da0s1b"           # Device name to crashdump to (or NO).
dumpdir="/usr/crash"