Surprising mention of unboxed integers in pattern exhaustiveness warning

The attached code produces this alarming warning:

~/waste$ ghc -Wall --make AlarmingPattern.hs -fforce-recomp
[1 of 1] Compiling Main             ( AlarmingPattern.hs, AlarmingPattern.o )

AlarmingPattern.hs:6:7: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘takeFromInv’:
        Patterns not matched:
            (GHC.Types.I# _) (GHC.Types.I# (#x)) with #x `notElem` [0#]
Linking AlarmingPattern ...

The error message is alarming because it refers to unboxed integers, but the source code does not.