[julia-users] Help understanding ASCIIString vs String

[Test This]

I defined a function 

function func(a::Params, b::String, c::Dict{String, Array{Int, 1}}, 
d::Dict{String, Array{Int, 1}})
  ...
end

When I run the program, calling this function with func(paramvalue, "H", 
d1, d2), I get an error saying func has no method matching
(::Params, ::ASCIIString, ::Dict{ASCIIString,Array{Int64,1}}, 
::Dict{ASCIIString,Array{Int64,1}})

The code works if I change String to ASCIIString in the function 
definition. But I thought (and the REPL seems to agree) that 
any value of type ASCIIString is also of type String. Then, why am I 
getting this error. 

Thanks in advance for your help.