ScalaCheck Output That I don't understand

scalacheck-output.txt

// The output.

info] ! Multiset.remove: Falsified after 1 passed tests.
[info] > ARG_0: (ListMultiset(, ),) <<<---- this is what looks weird, but I guess it's a ListMultiset with "" (no space)," " (a space) and the string is "" (no space) 
[info] ! Multiset.remove: Falsified after 7 passed tests.
[info] > ARG_0: (ListMultiset(),)

// The specification 

property("remove") = forAll(genMultisetWithString) { ((ms: Multiset[String], s: String) =>
  (ms - s).size == ms.size - 1
).tupled }

property("remove") = forAll(genMultisetWithString) { ((ms: Multiset[String], s: String) =>
  (ms - s).multiplicity(s) == ms.multiplicity(s) - 1
).tupled }

// The generator

val genMultisetWithString = for {
  mset <- genStringMultiset
  s <- Arbitrary.arbitrary[String]
} yield (mset + s, s)

The code seems to work in the REPL though :-/

scala> val ms = ListMultiset("", " ")
ms: com.sidewayscoding.immutable.ListMultiset[java.lang.String] = ListMultiset(,  )

scala> val s = ""
s: java.lang.String = ""

scala> (ms - s).size == ms.size - 1
res2: Boolean = true