`replace!` on a string gives an unfriendly internal error

[IanButterworth]

julia> str = "abc";

julia> replace!(str, "a" => "1")
ERROR: MethodError: no method matching _replace!(::Base.var"#new#replace_pairs!##0"{Tuple{Pair{String, String}}}, ::String, ::String, ::Int64)
The function `_replace!` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  _replace!(::Union{Function, Type}, ::Set{T}, ::AbstractSet, ::Int64) where T
   @ Base set.jl:985
  _replace!(::Union{Function, Type}, ::Dict{K, V}, ::AbstractDict, ::Int64) where {K, V}
   @ Base set.jl:953
  _replace!(::Union{Function, Type}, ::AbstractArray, ::AbstractArray, ::Int64)
   @ Base set.jl:920
  ...

Stacktrace:
 [1] replace_pairs!(res::String, A::String, count::Int64, old_new::Tuple{Pair{String, String}})
   @ Base ./set.jl:745
 [2] replace!(A::String, old_new::Pair{String, String}; count::Int64)
   @ Base ./set.jl:735
 [3] top-level scope
   @ REPL[18]:1