StrPack fails to unpack

[kuldeepdhaka]

using StrPack

@struct type Apple
    sweet::Float32
end

@struct type Boy
    eat::Apple
end

@struct type Cat
    home::Boy
end

c = Cat(Boy(Apple(0f0)))

ib = IOBuffer()
pack(ib, c)

seek(ib, 0)
unpack(ib, Cat)

results:

ERROR: LoadError: MethodError: Cannot `convert` an object of type Type{Apple} to an object of type StrPack.Struct
This may have arisen from a call to the constructor StrPack.Struct(...),
since type constructors fall back to convert methods.
Closest candidates are:
  StrPack.Struct(::Any, !Matched::Any, !Matched::Any)
  call{T}(::Type{T}, ::Any)
  convert{T}(::Type{T}, !Matched::T)
  ...
 in calcsize at /home/kuldeep/.julia/v0.5/StrPack/src/StrPack.jl:342
 [inlined code] from dict.jl:723
 in calcsize at /home/kuldeep/.julia/v0.5/StrPack/src/StrPack.jl:350
 in unpack at /home/kuldeep/.julia/v0.5/StrPack/src/StrPack.jl:175
 in unpack at /home/kuldeep/.julia/v0.5/StrPack/src/StrPack.jl:196
 in include at ./boot.jl:262
 in include_from_node1 at ./loading.jl:401
 in process_options at ./client.jl:245
 in _start at ./client.jl:301
while loading /home/kuldeep/Desktop/break.jl, in expression starting on line 21

can anybody explain why it failed?