diff --git a/base/strings/string.jl b/base/strings/string.jl
index 17d06fc464cc2..50b95f8cbf9f3 100644
--- a/base/strings/string.jl
+++ b/base/strings/string.jl
@@ -159,9 +159,7 @@ end
 
 ## checking UTF-8 & ACSII validity ##
 
-byte_string_classify(data::Vector{UInt8}) =
-    ccall(:u8_isvalid, Int32, (Ptr{UInt8}, Int), data, length(data))
-byte_string_classify(s::String) =
+byte_string_classify(s::Union{String,Vector{UInt8}}) =
     ccall(:u8_isvalid, Int32, (Ptr{UInt8}, Int), s, sizeof(s))
     # 0: neither valid ASCII nor UTF-8
     # 1: valid ASCII
diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index b18d63c9e1388..546fd6ea6e0dc 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -82,6 +82,12 @@ function isvalid(s::SubString, i::Integer)
     @inbounds return ib && isvalid(s.string, s.offset + i)
 end
 
+byte_string_classify(s::SubString{String}) =
+    ccall(:u8_isvalid, Int32, (Ptr{UInt8}, Int), s, sizeof(s))
+
+isvalid(::Type{String}, s::SubString{String}) = byte_string_classify(s) ≠ 0
+isvalid(s::SubString{String}) = isvalid(String, s)
+
 thisind(s::SubString{String}, i::Int) = _thisind_str(s, i)
 nextind(s::SubString{String}, i::Int) = _nextind_str(s, i)
 
diff --git a/base/strings/unicode.jl b/base/strings/unicode.jl
index e0c932a28f437..e5bd5e85f982a 100644
--- a/base/strings/unicode.jl
+++ b/base/strings/unicode.jl
@@ -12,13 +12,16 @@ import Base: show, ==, hash, string, Symbol, isless, length, eltype,
     isvalid(value) -> Bool
 
 Returns `true` if the given value is valid for its type, which currently can be either
-`AbstractChar` or `String`.
+`AbstractChar` or `String` or `SubString{String}`.
 
 # Examples
 ```jldoctest
 julia> isvalid(Char(0xd800))
 false
 
+julia> isvalid(SubString(String(UInt8[0xfe,0x80,0x80,0x80,0x80,0x80]),1,2))
+false
+
 julia> isvalid(Char(0xd799))
 true
 ```
@@ -30,13 +33,16 @@ isvalid(value)
 
 Returns `true` if the given value is valid for that type. Types currently can
 be either `AbstractChar` or `String`. Values for `AbstractChar` can be of type `AbstractChar` or [`UInt32`](@ref).
-Values for `String` can be of that type, or `Vector{UInt8}`.
+Values for `String` can be of that type, or `Vector{UInt8}` or `SubString{String}`.
 
 # Examples
 ```jldoctest
 julia> isvalid(Char, 0xd800)
 false
 
+julia> isvalid(String, SubString("thisisvalid",1,5))
+true
+
 julia> isvalid(Char, 0xd799)
 true
 ```
diff --git a/test/strings/basic.jl b/test/strings/basic.jl
index 150bbcfe48a4e..af17c4dee095a 100644
--- a/test/strings/basic.jl
+++ b/test/strings/basic.jl
@@ -523,7 +523,8 @@ end
     end
     # Check seven-byte sequences, should be invalid
     @test isvalid(String, UInt8[0xfe, 0x80, 0x80, 0x80, 0x80, 0x80]) == false
-
+    @test isvalid(lstrip("blablabla")) == true
+    @test isvalid(SubString(String(UInt8[0xfe, 0x80, 0x80, 0x80, 0x80, 0x80]), 1,2)) == false
     # invalid Chars
     @test  isvalid('a')
     @test  isvalid('柒')