diff --git a/src/read.jl b/src/read.jl
index 50354d4..b689f61 100644
--- a/src/read.jl
+++ b/src/read.jl
@@ -4,6 +4,7 @@ struct VectorString{T <: AbstractVector{UInt8}} <: AbstractString
 end
 
 Base.codeunits(x::VectorString) = x.bytes
+read_json_str(json::VectorString) = json
 
 # high-level user API functions
 read(io::Union{IO, Base.AbstractCmd}; kw...) = read(Base.read(io, String); kw...)
diff --git a/src/utils.jl b/src/utils.jl
index 63fa16c..8609047 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -192,7 +192,18 @@ end
 function read_json_str(json)
     # length check is to ensure that isfile doesn't thrown an error
     # see issue for details https://github.com/JuliaLang/julia/issues/39774
-    !(json isa VectorString) && sizeof(json) < 255 && isfile(json) ?
-          VectorString(Mmap.mmap(json)) : 
-          json
+    isfilename(json) && isfile(json) ?
+        VectorString(Mmap.mmap(json)) :
+        json
+end
+
+isfilename(filename) = try
+    stat(filename)
+    true
+catch e
+    if isa(e, Base.IOError)
+        e.code != -36   # -36 is ENAMETOOLONG; other IO errors indicate valid filename
+    else
+        rethrow()
+    end
 end