Using tuples has keys in Julia without initializing the dictionary - dictionary

Question

I am new to Julia. For my application I have an expensive operation to calculate a matrix based on two points. Calculated matrices will be used multiple times so I though I would cache them in a dictionary. However, I have encountered a strange behaviour in Julia. If I add a first (key, value) pair as part of the dictionary initialization it all works as expected. However, if I don't, and try and add my first (key, value) pair later, Julia bombs out.
p1 = [ 2.0, 1.0, 1.0]
p2 = [ 4.0, 2.0, 1.0]
# This works
my_dict = Dict{Tuple{Array{Float64,1}, Array{Float64,1}}, String}(
(p1, p2) => "hello world"
)
println(my_dict[(p1, p2)])
# This works
my_dict[ (p2, p1)] = "hello again"
println(my_dict[ (p2, p1)] )
# This doesn't
my_dict2 = Dict{Tuple{Array{Float64,1}, Array{Float64,1}}, String}
my_dict2[(p2, p1)] = "this fails with method dispatch error"
Output from Julia:
julia complex_key_for_dict.jl
hello world
hello again
ERROR: LoadError: MethodError: no method matching setindex!(::Type{Dict{Tuple{Array{Float64,1},Array{Float64,1}},String}}, ::String, ::Tuple{Array{Float64,1},Array{Float64,1}})
Stacktrace:
[1] top-level scope at none:0
[2] include at ./boot.jl:326 [inlined]
[3] include_relative(::Module, ::String) at ./loading.jl:1038
[4] include(::Module, ::String) at ./sysimg.jl:29
[5] exec_options(::Base.JLOptions) at ./client.jl:267
[6] _start() at ./client.jl:436
in expression starting at /home/peter/julia/gps_analysis/complex_key_for_dict.jl:16
For now, I will simply add a fictitious first entry into the dictionary, but would be grateful if someone can put me straight on my error.

Answer 1

Nothing like posting a question to StackOverflow to me make realise what the solution might be. This works:
# This works. Added an open/close bracket to the end of the Dict defintion
my_dict2 = Dict{Tuple{Array{Float64,1}, Array{Float64,1}}, String}()
my_dict2[ (p2, p1)] = "this now works"
println(my_dict2[ (p2, p1)])
Thought I would post the answer so that anyone else who has also banged their head against a brick wall on this may find it helpful.