diff --git a/src/pkg/mime/multipart/multipart_test.go b/src/pkg/mime/multipart/multipart_test.go
index 3f3b8b1a0e7..38079e53a19 100644
--- a/src/pkg/mime/multipart/multipart_test.go
+++ b/src/pkg/mime/multipart/multipart_test.go
@@ -141,14 +141,14 @@ func testMultipart(t *testing.T, r io.Reader, onlyNewlines bool) {
 		t.Error("Expected part1")
 		return
 	}
-	if part.Header.Get("Header1") != "value1" {
-		t.Error("Expected Header1: value")
+	if x := part.Header.Get("Header1"); x != "value1" {
+		t.Errorf("part.Header.Get(%q) = %q, want %q", "Header1", x, "value1")
 	}
-	if part.Header.Get("foo-bar") != "baz" {
-		t.Error("Expected foo-bar: baz")
+	if x := part.Header.Get("foo-bar"); x != "baz" {
+		t.Errorf("part.Header.Get(%q) = %q, want %q", "foo-bar", x, "baz")
 	}
-	if part.Header.Get("Foo-Bar") != "baz" {
-		t.Error("Expected Foo-Bar: baz")
+	if x := part.Header.Get("Foo-Bar"); x != "baz" {
+		t.Errorf("part.Header.Get(%q) = %q, want %q", "Foo-Bar", x, "baz")
 	}
 	buf.Reset()
 	if _, err := io.Copy(buf, part); err != nil {
diff --git a/src/pkg/net/textproto/reader.go b/src/pkg/net/textproto/reader.go
index 9b5befe9aa5..6031baa3bb2 100644
--- a/src/pkg/net/textproto/reader.go
+++ b/src/pkg/net/textproto/reader.go
@@ -115,6 +115,13 @@ func (r *Reader) readContinuedLineSlice() ([]byte, os.Error) {
 	}
 	line = trim(line)
 
+	copied := false
+	if r.R.Buffered() < 1 {
+		// ReadByte will flush the buffer; make a copy of the slice.
+		copied = true
+		line = append([]byte(nil), line...)
+	}
+
 	// Look for a continuation line.
 	c, err := r.R.ReadByte()
 	if err != nil {
@@ -127,6 +134,11 @@ func (r *Reader) readContinuedLineSlice() ([]byte, os.Error) {
 		return line, nil
 	}
 
+	if !copied {
+		// The next readLineSlice will invalidate the previous one.
+		line = append(make([]byte, 0, len(line)*2), line...)
+	}
+
 	// Read continuation lines.
 	for {
 		// Consume leading spaces; one already gone.
@@ -140,9 +152,6 @@ func (r *Reader) readContinuedLineSlice() ([]byte, os.Error) {
 				break
 			}
 		}
-		// copy now since the next call to read a slice invalidates line
-		line = append(make([]byte, 0, len(line)*2), line...)
-
 		var cont []byte
 		cont, err = r.readLineSlice()
 		cont = trim(cont)