Only one uri instance allowed

[mds-humanoid]

When I start a fresh slime I seem to be able to create as many uri instances as I want with something like:
(defvar uri5 (uri "http://www.ics.uci2.edu/pub/ietf/uri2/#Related"))

However, after I load my project, which has #:quri in the depends-on list in its .asd file, I can no longer instantiate another uri instance without getting the following error:

invalid number of arguments: 5
[Condition of type SB-INT:SIMPLE-PROGRAM-ERROR]

Restarts:
0: [*ABORT] Return to SLIME's top level.
1: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {1004EB0033}>)

Backtrace:
0: (PARSE-URI "http://www.ics.uci2.edu/pub/ietf/uri2/#Related" :START 0 :END NIL) [tl,external]
1: (URI "http://www.ics.uci2.edu/pub/ietf/uri2/#Related" :START 0 :END NIL)

[fukamachi]

How is this related to prove?
What are the minimum steps to reproduce it?