diff --git a/src/pkg/io/io.go b/src/pkg/io/io.go
index 859adaf1b71..5b24f062efc 100644
--- a/src/pkg/io/io.go
+++ b/src/pkg/io/io.go
@@ -262,6 +262,7 @@ func WriteString(w Writer, s string) (n int, err error) {
 // If an EOF happens after reading fewer than min bytes,
 // ReadAtLeast returns ErrUnexpectedEOF.
 // If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
+// On return, n >= min if and only if err == nil.
 func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
 	if len(buf) < min {
 		return 0, ErrShortBuffer
@@ -271,12 +272,10 @@ func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
 		nn, err = r.Read(buf[n:])
 		n += nn
 	}
-	if err == EOF {
-		if n >= min {
-			err = nil
-		} else if n > 0 {
-			err = ErrUnexpectedEOF
-		}
+	if n >= min {
+		err = nil
+	} else if n > 0 && err == EOF {
+		err = ErrUnexpectedEOF
 	}
 	return
 }
@@ -286,6 +285,7 @@ func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
 // The error is EOF only if no bytes were read.
 // If an EOF happens after reading some but not all the bytes,
 // ReadFull returns ErrUnexpectedEOF.
+// On return, n == len(buf) if and only if err == nil.
 func ReadFull(r Reader, buf []byte) (n int, err error) {
 	return ReadAtLeast(r, buf, len(buf))
 }
diff --git a/src/pkg/io/io_test.go b/src/pkg/io/io_test.go
index f3ec050fad9..307066825ba 100644
--- a/src/pkg/io/io_test.go
+++ b/src/pkg/io/io_test.go
@@ -6,6 +6,7 @@ package io_test
 
 import (
 	"bytes"
+	"fmt"
 	. "io"
 	"strings"
 	"testing"
@@ -120,22 +121,30 @@ func TestReadAtLeast(t *testing.T) {
 	testReadAtLeast(t, &rb)
 }
 
-// A version of bytes.Buffer that returns n > 0, EOF on Read
+// A version of bytes.Buffer that returns n > 0, err on Read
 // when the input is exhausted.
-type dataAndEOFBuffer struct {
+type dataAndErrorBuffer struct {
+	err error
 	bytes.Buffer
 }
 
-func (r *dataAndEOFBuffer) Read(p []byte) (n int, err error) {
+func (r *dataAndErrorBuffer) Read(p []byte) (n int, err error) {
 	n, err = r.Buffer.Read(p)
 	if n > 0 && r.Buffer.Len() == 0 && err == nil {
-		err = EOF
+		err = r.err
 	}
 	return
 }
 
 func TestReadAtLeastWithDataAndEOF(t *testing.T) {
-	var rb dataAndEOFBuffer
+	var rb dataAndErrorBuffer
+	rb.err = EOF
+	testReadAtLeast(t, &rb)
+}
+
+func TestReadAtLeastWithDataAndError(t *testing.T) {
+	var rb dataAndErrorBuffer
+	rb.err = fmt.Errorf("fake error")
 	testReadAtLeast(t, &rb)
 }
 
@@ -169,8 +178,12 @@ func testReadAtLeast(t *testing.T, rb ReadWriter) {
 	}
 	rb.Write([]byte("4"))
 	n, err = ReadAtLeast(rb, buf, 2)
-	if err != ErrUnexpectedEOF {
-		t.Errorf("expected ErrUnexpectedEOF, got %v", err)
+	want := ErrUnexpectedEOF
+	if rb, ok := rb.(*dataAndErrorBuffer); ok && rb.err != EOF {
+		want = rb.err
+	}
+	if err != want {
+		t.Errorf("expected %v, got %v", want, err)
 	}
 	if n != 1 {
 		t.Errorf("expected to have read 1 bytes, got %v", n)