diff --git a/src/pkg/archive/tar/reader.go b/src/pkg/archive/tar/reader.go
index d15e120792..e6ac538aad 100644
--- a/src/pkg/archive/tar/reader.go
+++ b/src/pkg/archive/tar/reader.go
@@ -468,14 +468,14 @@ func (tr *Reader) readHeader() *Header {
 	// so its magic bytes, like the rest of the block, are NULs.
 	magic := string(s.next(8)) // contains version field as well.
 	var format string
-	switch magic {
-	case "ustar\x0000": // POSIX tar (1003.1-1988)
+	switch {
+	case magic[:6] == "ustar\x00": // POSIX tar (1003.1-1988)
 		if string(header[508:512]) == "tar\x00" {
 			format = "star"
 		} else {
 			format = "posix"
 		}
-	case "ustar  \x00": // old GNU tar
+	case magic == "ustar  \x00": // old GNU tar
 		format = "gnu"
 	}
 
diff --git a/src/pkg/archive/tar/testdata/writer-big-long.tar b/src/pkg/archive/tar/testdata/writer-big-long.tar
new file mode 100644
index 0000000000..5960ee8247
Binary files /dev/null and b/src/pkg/archive/tar/testdata/writer-big-long.tar differ
diff --git a/src/pkg/archive/tar/writer.go b/src/pkg/archive/tar/writer.go
index 9ee9499297..6eff6f6f84 100644
--- a/src/pkg/archive/tar/writer.go
+++ b/src/pkg/archive/tar/writer.go
@@ -218,8 +218,8 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 				tw.cString(prefixHeaderBytes, prefix, false, paxNone, nil)
 
 				// Use the ustar magic if we used ustar long names.
-				if len(prefix) > 0 {
-					copy(header[257:265], []byte("ustar\000"))
+				if len(prefix) > 0 && !tw.usedBinary {
+					copy(header[257:265], []byte("ustar\x00"))
 				}
 			}
 		}
diff --git a/src/pkg/archive/tar/writer_test.go b/src/pkg/archive/tar/writer_test.go
index 2b9ea658db..512fab1a6f 100644
--- a/src/pkg/archive/tar/writer_test.go
+++ b/src/pkg/archive/tar/writer_test.go
@@ -103,6 +103,29 @@ var writerTests = []*writerTest{
 			},
 		},
 	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > (longname/)*15 /16gig.txt
+	//   tar -b 1 -c -f- (longname/)*15 /16gig.txt | dd bs=512 count=8 > writer-big-long.tar
+	{
+		file: "testdata/writer-big-long.tar",
+		entries: []*writerTestEntry{
+			{
+				header: &Header{
+					Name:     strings.Repeat("longname/", 15) + "16gig.txt",
+					Mode:     0644,
+					Uid:      1000,
+					Gid:      1000,
+					Size:     16 << 30,
+					ModTime:  time.Unix(1399583047, 0),
+					Typeflag: '0',
+					Uname:    "guillaume",
+					Gname:    "guillaume",
+				},
+				// fake contents
+				contents: strings.Repeat("\x00", 4<<10),
+			},
+		},
+	},
 	// This file was produced using gnu tar 1.17
 	// gnutar  -b 4 --format=ustar (longname/)*15 + file.txt
 	{