diff --git a/lib/protocol/bep_extensions.go b/lib/protocol/bep_extensions.go
index bc0b41bae..62be831ea 100644
--- a/lib/protocol/bep_extensions.go
+++ b/lib/protocol/bep_extensions.go
@@ -324,7 +324,7 @@ func (b BlockInfo) IsEmpty() bool {
 type IndexID uint64
 
 func (i IndexID) String() string {
-	return fmt.Sprintf("0x%16X", uint64(i))
+	return fmt.Sprintf("0x%016X", uint64(i))
 }
 
 func (i IndexID) Marshal() ([]byte, error) {
diff --git a/lib/protocol/protocol_test.go b/lib/protocol/protocol_test.go
index 9e71785b2..380dddb2a 100644
--- a/lib/protocol/protocol_test.go
+++ b/lib/protocol/protocol_test.go
@@ -915,3 +915,11 @@ func TestBlocksEqual(t *testing.T) {
 		}
 	}
 }
+
+func TestIndexIDString(t *testing.T) {
+	// Index ID is a 64 bit, zero padded hex integer.
+	var i IndexID = 42
+	if i.String() != "0x000000000000002A" {
+		t.Error(i.String())
+	}
+}