diff --git a/src/buffer.jl b/src/buffer.jl
index 8e5d744..987ff05 100644
--- a/src/buffer.jl
+++ b/src/buffer.jl
@@ -203,7 +203,7 @@ end
 
 # Insert data to the current buffer.
 # `data` must not alias `buf`
-function insertdata!(buf::Buffer, data::Union{AbstractArray{UInt8}, Memory})
+function insertdata!(buf::Buffer, data::Union{AbstractVector{UInt8}, Memory})
     nbytes = Int(length(data))
     makemargin!(buf, nbytes)
     datapos = if iszero(buf.markpos)
diff --git a/src/stream.jl b/src/stream.jl
index d057cf3..8114157 100644
--- a/src/stream.jl
+++ b/src/stream.jl
@@ -462,6 +462,7 @@ inserted.
 `data` must not alias any internal buffers in `stream`
 """
 function unread(stream::TranscodingStream, data::AbstractVector{UInt8})
+    ready_to_read!(stream)
     insertdata!(stream.buffer1, data)
     return nothing
 end
diff --git a/test/codecdoubleframe.jl b/test/codecdoubleframe.jl
index 7730d8a..c39fdbc 100644
--- a/test/codecdoubleframe.jl
+++ b/test/codecdoubleframe.jl
@@ -396,6 +396,22 @@ DoubleFrameDecoderStream(stream::IO; kwargs...) = TranscodingStream(DoubleFrameD
         close(stream)
     end
 
+    @testset "unread" begin
+        stream = DoubleFrameDecoderStream(IOBuffer("[ ffoooobbaarr ]"))
+        @test position(stream) == 0
+        @test read(stream, 3) == b"foo"
+        @test position(stream) == 3
+        @test read(stream, 3) == b"bar"
+        @test position(stream) == 6
+        @test TranscodingStreams.unread(stream, b"baz") === nothing
+        @test position(stream) == 3
+        @test read(stream, 3) == b"baz"
+        @test position(stream) == 6
+        @test eof(stream)
+        @test position(stream) == 6
+        close(stream)
+    end
+
     test_roundtrip_read(DoubleFrameEncoderStream, DoubleFrameDecoderStream)
     test_roundtrip_write(DoubleFrameEncoderStream, DoubleFrameDecoderStream)
     test_roundtrip_lines(DoubleFrameEncoderStream, DoubleFrameDecoderStream)
diff --git a/test/codecnoop.jl b/test/codecnoop.jl
index 9db6bc1..d5a0862 100644
--- a/test/codecnoop.jl
+++ b/test/codecnoop.jl
@@ -325,13 +325,20 @@ using FillArrays: Zeros
     @testset "unread" begin
         stream = NoopStream(IOBuffer(""))
         @test TranscodingStreams.unread(stream, b"foo") === nothing
+        @test position(stream) == -3
         @test read(stream, 3) == b"foo"
+        @test position(stream) == 0
+        @test eof(stream)
         close(stream)
 
         stream = NoopStream(IOBuffer("foo"))
         @test read(stream, 3) == b"foo"
+        @test position(stream) == 3
         @test TranscodingStreams.unread(stream, b"bar") === nothing
+        @test position(stream) == 0
         @test read(stream, 3) == b"bar"
+        @test position(stream) == 3
+        @test eof(stream)
         close(stream)
 
         stream = NoopStream(IOBuffer("foobar"))
@@ -351,24 +358,36 @@ using FillArrays: Zeros
         close(stream)
 
         stream = NoopStream(IOBuffer("foobar"))
+        @test position(stream) == 0
         @test read(stream, 3) == b"foo"
+        @test position(stream) == 3
         @test read(stream, 3) == b"bar"
+        @test position(stream) == 6
         @test TranscodingStreams.unread(stream, b"baz") === nothing
+        @test position(stream) == 3
         @test read(stream, 3) == b"baz"
+        @test position(stream) == 6
         @test eof(stream)
+        @test position(stream) == 6
         close(stream)
 
         for bufsize in (1, 2, 3, 4, 100)
             for n in (1, 100)
                 stream = NoopStream(IOBuffer("foo"^n*"bar"^n); bufsize)
                 @test mark(stream) == 0
+                @test position(stream) == 0
                 @test read(stream, 3n) == codeunits("foo"^n)
                 @test read(stream, 3n) == codeunits("bar"^n)
+                @test position(stream) == 6n
                 TranscodingStreams.unread(stream, codeunits("baz"^n))
+                @test position(stream) == 3n
                 @test reset(stream) == 0
+                @test position(stream) == 0
                 @test read(stream, 3n) == codeunits("foo"^n)
                 @test read(stream, 3n) == codeunits("baz"^n)
+                @test position(stream) == 6n
                 @test eof(stream)
+                @test position(stream) == 6n
                 close(stream)
             end
         end
@@ -414,6 +433,11 @@ using FillArrays: Zeros
         end
         @test read(stream, 3) == b"bar"
         close(stream)
+
+        stream = NoopStream(IOBuffer())
+        write(stream, b"foo")
+        @test_throws ArgumentError TranscodingStreams.unread(stream, b"bar")
+        close(stream)
     end
 
     stream = NoopStream(IOBuffer(""))