Default values for nested struct array

New to Julia
1

I can’t work out how to specify default values for a struct array that is nested. I have one struct a_t which specifies default values. I have a MVector of type a_t in the struct b_t. The following code fails. I want the MVector b2 of type a_t to take the default values already specified. (Note that if I did not have a vector, e.g. b2::a_t = a_t(), then that works as expected.)

using StaticArrays
using Parameters

@with_kw mutable struct a_t
    a1::Int64 = 10
    a2::Float64 = 0.0
end

@with_kw mutable struct b_t
    b1::Float64 = 0.0
    b2::MVector{2, a_t} .= a_t()
end
2 
Base.@kwdef mutable struct b_t
    b1::Float64 = 0.0
    b2::MVector{2, a_t} = @MVector [a_t() for _ = 1:2]
end