diff --git a/internal/packindex/content_id_to_bytes.go b/internal/packindex/content_id_to_bytes.go
index bc123a2a6..731f3b57d 100644
--- a/internal/packindex/content_id_to_bytes.go
+++ b/internal/packindex/content_id_to_bytes.go
@@ -21,14 +21,15 @@ func bytesToContentID(b []byte) string {
 
 func contentIDToBytes(c string) []byte {
 	var prefix []byte
+	var skip int
 	if len(c)%2 == 1 {
 		prefix = []byte(c[0:1])
-		c = c[1:]
+		skip = 1
 	} else {
 		prefix = []byte{0}
 	}
 
-	b, err := hex.DecodeString(c)
+	b, err := hex.DecodeString(c[skip:])
 	if err != nil {
 		return append([]byte{0xff}, []byte(c)...)
 	}
diff --git a/internal/packindex/index.go b/internal/packindex/index.go
index ab31e7d19..1ea4c2cb2 100644
--- a/internal/packindex/index.go
+++ b/internal/packindex/index.go
@@ -7,21 +7,18 @@
 	"io"
 	"sort"
 	"strings"
-	"sync"
 )
 
 // Index is a read-only index of packed blocks.
 type Index interface {
 	io.Closer
 
-	EntryCount() int
 	GetInfo(blockID string) (*Info, error)
 	Iterate(prefix string, cb func(Info) error) error
 }
 
 type index struct {
 	hdr      headerInfo
-	mu       sync.Mutex
 	readerAt io.ReaderAt
 }
 
@@ -42,25 +39,23 @@ func readHeader(readerAt io.ReaderAt) (headerInfo, error) {
 		return headerInfo{}, fmt.Errorf("invalid header format: %v", header[0])
 	}
 
-	return headerInfo{
+	hi := headerInfo{
 		keySize:    int(header[1]),
 		valueSize:  int(binary.BigEndian.Uint16(header[2:4])),
 		entryCount: int(binary.BigEndian.Uint32(header[4:8])),
-	}, nil
-}
+	}
 
-// EntryCount returns the number of block entries in an index.
-func (b *index) EntryCount() int {
-	return b.hdr.entryCount
+	if hi.keySize <= 1 || hi.valueSize < 0 || hi.entryCount < 0 {
+		return headerInfo{}, fmt.Errorf("invalid header")
+	}
+
+	return hi, nil
 }
 
 // Iterate invokes the provided callback function for all blocks in the index, sorted alphabetically.
 // The iteration ends when the callback returns an error, which is propagated to the caller or when
 // all blocks have been visited.
 func (b *index) Iterate(prefix string, cb func(Info) error) error {
-	b.mu.Lock()
-	defer b.mu.Unlock()
-
 	startPos, err := b.findEntryPosition(prefix)
 	if err != nil {
 		return fmt.Errorf("could not find starting position: %v", err)
@@ -139,9 +134,6 @@ func (b *index) findEntry(blockID string) ([]byte, error) {
 
 // GetInfo returns information about a given block. If a block is not found, nil is returned.
 func (b *index) GetInfo(blockID string) (*Info, error) {
-	b.mu.Lock()
-	defer b.mu.Unlock()
-
 	e, err := b.findEntry(blockID)
 	if err != nil {
 		return nil, err
diff --git a/internal/packindex/merged.go b/internal/packindex/merged.go
index a1aad2104..76e637972 100644
--- a/internal/packindex/merged.go
+++ b/internal/packindex/merged.go
@@ -19,15 +19,6 @@ func (m Merged) Close() error {
 	return nil
 }
 
-// EntryCount returns the cumulative number of entries in all underlying indexes (not necessarily the number of unique block IDs).
-func (m Merged) EntryCount() int {
-	cnt := 0
-	for _, ndx := range m {
-		cnt += ndx.EntryCount()
-	}
-	return cnt
-}
-
 // GetInfo returns information about a single block. If a block is not found, returns (nil,nil)
 func (m Merged) GetInfo(contentID string) (*Info, error) {
 	var best *Info
diff --git a/internal/packindex/merged_test.go b/internal/packindex/merged_test.go
index f5a31e252..8ff02abe0 100644
--- a/internal/packindex/merged_test.go
+++ b/internal/packindex/merged_test.go
@@ -75,6 +75,10 @@ func TestMerged(t *testing.T) {
 	if !reflect.DeepEqual(inOrder, expectedInOrder) {
 		t.Errorf("unexpected items in order: %v, wanted %v", inOrder, expectedInOrder)
 	}
+
+	if err := m.Close(); err != nil {
+		t.Errorf("unexpected error in Close(): %v", err)
+	}
 }
 
 func indexWithItems(items ...packindex.Info) (packindex.Index, error) {
diff --git a/internal/packindex/packindex_internal_test.go b/internal/packindex/packindex_internal_test.go
new file mode 100644
index 000000000..a80661e82
--- /dev/null
+++ b/internal/packindex/packindex_internal_test.go
@@ -0,0 +1,26 @@
+package packindex
+
+import "testing"
+
+func TestRoundTrip(t *testing.T) {
+	cases := []string{
+		"",
+		"x",
+		"aa",
+		"xaa",
+		"xaaa",
+		"a1x",
+	}
+
+	for _, tc := range cases {
+		b := contentIDToBytes(tc)
+		got := bytesToContentID(b)
+		if got != tc {
+			t.Errorf("%q did not round trip, got %q, wanted %q", tc, got, tc)
+		}
+	}
+
+	if got, want := bytesToContentID(nil), ""; got != want {
+		t.Errorf("unexpected content id %v, want %v", got, want)
+	}
+}
diff --git a/internal/packindex/packindex_test.go b/internal/packindex/packindex_test.go
index b4e3b7f94..821f5d08f 100644
--- a/internal/packindex/packindex_test.go
+++ b/internal/packindex/packindex_test.go
@@ -119,10 +119,16 @@ func TestPackIndex(t *testing.T) {
 		t.Errorf("builder output not stable: %x vs %x", hex.Dump(data2), hex.Dump(data3))
 	}
 
+	t.Run("FuzzTest", func(t *testing.T) {
+		fuzzTestIndexOpen(t, data1)
+	})
+
 	ndx, err := packindex.Open(bytes.NewReader(data1))
 	if err != nil {
 		t.Fatalf("can't open index: %v", err)
 	}
+	defer ndx.Close()
+
 	for _, info := range infos {
 		info2, err := ndx.GetInfo(info.BlockID)
 		if err != nil {
@@ -172,3 +178,60 @@ func TestPackIndex(t *testing.T) {
 		t.Logf("found %v elements with prefix %q", cnt2, prefix)
 	}
 }
+
+func fuzzTestIndexOpen(t *testing.T, originalData []byte) {
+	// use consistent random
+	rnd := rand.New(rand.NewSource(12345))
+
+	fuzzTest(rnd, originalData, 50000, func(d []byte) {
+		ndx, err := packindex.Open(bytes.NewReader(d))
+		if err != nil {
+			return
+		}
+		defer ndx.Close()
+		cnt := 0
+		ndx.Iterate("", func(cb packindex.Info) error {
+			if cnt < 10 {
+				ndx.GetInfo(cb.BlockID)
+			}
+			cnt++
+			return nil
+		})
+	})
+}
+
+func fuzzTest(rnd *rand.Rand, originalData []byte, rounds int, callback func(d []byte)) {
+	for round := 0; round < rounds; round++ {
+		data := append([]byte(nil), originalData...)
+
+		// mutate small number of bytes
+		bytesToMutate := rnd.Intn(3)
+		for i := 0; i < bytesToMutate; i++ {
+			pos := rnd.Intn(len(data))
+			data[pos] = byte(rnd.Int())
+		}
+
+		sectionsToInsert := rnd.Intn(3)
+		for i := 0; i < sectionsToInsert; i++ {
+			pos := rnd.Intn(len(data))
+			insertedLength := rnd.Intn(20)
+			insertedData := make([]byte, insertedLength)
+			rnd.Read(insertedData)
+
+			data = append(append(append([]byte(nil), data[0:pos]...), insertedData...), data[pos:]...)
+		}
+
+		sectionsToDelete := rnd.Intn(3)
+		for i := 0; i < sectionsToDelete; i++ {
+			pos := rnd.Intn(len(data))
+			deletedLength := rnd.Intn(10)
+			if pos+deletedLength > len(data) {
+				continue
+			}
+
+			data = append(append([]byte(nil), data[0:pos]...), data[pos+deletedLength:]...)
+		}
+
+		callback(data)
+	}
+}
diff --git a/internal/packindex/subset.go b/internal/packindex/subset.go
index 5946ade6c..5c6e55299 100644
--- a/internal/packindex/subset.go
+++ b/internal/packindex/subset.go
@@ -2,11 +2,6 @@
 
 // IsSubset returns true if all entries in index 'a' are contained in index 'b'.
 func IsSubset(a, b Index) bool {
-	if a.EntryCount() > b.EntryCount() {
-		// no point iterating.
-		return false
-	}
-
 	done := make(chan bool)
 	defer close(done)
 
diff --git a/repo/block/committed_block_index.go b/repo/block/committed_block_index.go
index 3d67f6d18..103866790 100644
--- a/repo/block/committed_block_index.go
+++ b/repo/block/committed_block_index.go
@@ -106,7 +106,6 @@ func (b *committedBlockIndex) use(packFiles []string) (bool, error) {
 			return false, fmt.Errorf("unable to open pack index %q: %v", e, err)
 		}
 
-		log.Debugf("opened %v with %v entries", e, ndx.EntryCount())
 		newMerged = append(newMerged, ndx)
 		newInUse[e] = ndx
 	}