CWM Bug: Strings Returned from Builtins Incompatible with log:semantics

The test:-

this log:forAll :p, :q, :r, :s .
{ (":x :y :z " ".") string:concatenation [ log:semantics :s ] .
   :s log:includes { :p :q :r } } log:implies { :p :q :r } .

the error raised:-

[...]
  File "/home/2000/10/swap/llyn.py", line 2299, in query2
    nbs = item.tryHeavy(queue, bindings)
  File "/home/2000/10/swap/llyn.py", line 2574, in tryHeavy
    raise BuiltInFailed(sys.exc_info(), self ),None
llyn.BuiltInFailed: Error during built-in operation
 20) short=0, 0_work ::  ":x :y :z" semantics        s(s)?.
because:
"    Unable to access document <md5:fd318d2e2af85d35e494e0f14d2401b9>,
because:\n        [Errno url error] unknown url type: 'md5'"

--
Kindest Regards,
Sean B. Palmer
@prefix : <http://purl.org/net/swn#> .
:Sean :homepage <http://purl.org/net/sbp/> .