Parametric type construction with Parameters.jl fails

I like how @with_kw can construct new type instances based on an existing one, and was successful with it until I made my struct parametric. First, the non-parametric version:

using Parameters
abstract type SomeTypes end 
makeinstance(mine::M; moreparms...) where M<:SomeTypes = M(mine; moreparms...)
@with_kw struct AnyType <: SomeTypes # lacking parametric types
    a = 1.
    b = 2.

m = AnyType();
makeinstance(m; b=3.) # works fine to change field b

But when the struct is parametric, the same thing works in the REPL but fails in the function:

@with_kw struct SameType{T} <: SomeTypes # with parametric type T
    a::T = 1.
    b::T = 2.
n = SameType();

julia> SameType(n; b=3.) # optionally changing b works in REPL
  a: Float64 1.0
  b: Float64 3.0

julia> makeinstance(n; b=3.) # but not in a function! Why?
ERROR: MethodError: no method matching SameType{Float64}(::SameType{Float64}; b::Float64)

Closest candidates are:
  SameType{T}(::Any, ::Any) where T got unsupported keyword argument "b"
   @ Main ~/.julia/packages/Parameters/MK0O4/src/Parameters.jl:503
  SameType{T}(; a, b) where T
   @ Main ~/.julia/packages/Parameters/MK0O4/src/Parameters.jl:491

 [1] makeinstance(mine::SameType{Float64}; moreparms::@Kwargs{b::Float64})
   @ Main ./REPL[8]:1
 [2] top-level scope
   @ REPL[16]:1

I don’t understand the MethodError, and why this doesn’t work parametrically. I’d appreciate any suggestions how to get this to work within the function. Thanks!