diff --git a/src/io/export_test.go b/src/io/export_test.go
new file mode 100644
index 0000000000..fa3e8e76f6
--- /dev/null
+++ b/src/io/export_test.go
@@ -0,0 +1,8 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io
+
+// exported for test
+var ErrInvalidWrite = errInvalidWrite
diff --git a/src/io/io.go b/src/io/io.go
index 4bd1ae913a..a34c39a32a 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -27,12 +27,12 @@ const (
 // but failed to return an explicit error.
 var ErrShortWrite = errors.New("short write")
 
+// errInvalidWrite means that a write returned an impossible count.
+var errInvalidWrite = errors.New("invalid write result")
+
 // ErrShortBuffer means that a read required a longer buffer than was provided.
 var ErrShortBuffer = errors.New("short buffer")
 
-// ErrBadWriteCount means that a write returned an impossible count.
-var ErrBadWriteCount = errors.New("Write returned impossible count")
-
 // EOF is the error returned by Read when no more input is available.
 // (Read must return EOF itself, not an error wrapping EOF,
 // because callers will test for EOF using ==.)
@@ -425,7 +425,7 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 			if nw < 0 || nr < nw {
 				nw = 0
 				if ew == nil {
-					ew = ErrBadWriteCount
+					ew = errInvalidWrite
 				}
 			}
 			written += int64(nw)
diff --git a/src/io/io_test.go b/src/io/io_test.go
index a8399bcac6..5b355e8c55 100644
--- a/src/io/io_test.go
+++ b/src/io/io_test.go
@@ -441,7 +441,7 @@ func (w largeWriter) Write(p []byte) (int, error) {
 }
 
 func TestCopyLargeWriter(t *testing.T) {
-	want := ErrBadWriteCount
+	want := ErrInvalidWrite
 	rb := new(Buffer)
 	wb := largeWriter{}
 	rb.WriteString("hello, world.")