Quoting Adam C. Emerson (aemerson@acm.org):
An attempt to access an array of element-type NIL was made. Congratulations! [Condition of type SB-KERNEL:NIL-ARRAY-ACCESSED-ERROR]
Heh.
Well, the puri library was apparently written by someone trying to maximize speed at all cost. It doesn't work with non-simple strings.
Here's a second attempt at the patch, including a workaround for that.
I'm also attaching an unrelated patch to puri, which you might need if you're running a very current SBCL. (puri uses sb-kernel:shrink-vector, which is not exactly a supported interface in SBCL and has recently changed behaviour.)