diff --git a/util/proto/marshal.go b/util/proto/marshal.go
index 250db0f..66824b3 100644
--- a/util/proto/marshal.go
+++ b/util/proto/marshal.go
@@ -335,3 +335,28 @@ func NestedStructureSize(field int64, v stableMarshaller) (size int) {
 
 	return size
 }
+
+func Fixed64Marshal(field int, buf []byte, v uint64) (int, error) {
+	if v == 0 {
+		return 0, nil
+	}
+
+	prefix := field<<3 | 1
+
+	// buf length check can prevent panic at PutUvarint, but it will make
+	// marshaller a bit slower.
+	i := binary.PutUvarint(buf, uint64(prefix))
+	binary.LittleEndian.PutUint64(buf[i:], v)
+
+	return i + 8, nil
+}
+
+func Fixed64Size(fNum int, v uint64) int {
+	if v == 0 {
+		return 0
+	}
+
+	prefix := fNum<<3 | 1
+
+	return VarUIntSize(uint64(prefix)) + 8
+}
diff --git a/util/proto/marshal_test.go b/util/proto/marshal_test.go
index 56e0a23..6844309 100644
--- a/util/proto/marshal_test.go
+++ b/util/proto/marshal_test.go
@@ -22,6 +22,7 @@ type stablePrimitives struct {
 	FieldF int64
 	FieldG uint64
 	FieldH SomeEnum
+	FieldI uint64 // fixed64
 }
 
 type stableRepPrimitives struct {
@@ -122,6 +123,16 @@ func (s *stablePrimitives) stableMarshal(buf []byte, wrongField bool) ([]byte, e
 	}
 	i += offset
 
+	fieldNum = 205
+	if wrongField {
+		fieldNum++
+	}
+	offset, err = proto.Fixed64Marshal(fieldNum, buf, s.FieldI)
+	if err != nil {
+		return nil, errors.Wrap(err, "can't marshal field I")
+	}
+	i += offset
+
 	fieldNum = 300
 	if wrongField {
 		fieldNum++
@@ -143,6 +154,7 @@ func (s *stablePrimitives) stableSize() int {
 		proto.UInt32Size(202, s.FieldE) +
 		proto.Int64Size(203, s.FieldF) +
 		proto.UInt64Size(204, s.FieldG) +
+		proto.Fixed64Size(205, s.FieldI) +
 		proto.EnumSize(300, int32(s.FieldH))
 }
 
@@ -430,6 +442,17 @@ func TestRepeatedUInt64Marshal(t *testing.T) {
 	})
 }
 
+func TestFixed64Marshal(t *testing.T) {
+	t.Run("zero", func(t *testing.T) {
+		testFixed64Marshal(t, 0, false)
+	})
+
+	t.Run("non zero", func(t *testing.T) {
+		testFixed64Marshal(t, math.MaxUint64, false)
+		testFixed64Marshal(t, math.MaxUint64, true)
+	})
+}
+
 func testMarshal(t *testing.T, c stablePrimitives, tr test.Primitives, wrongField bool) *test.Primitives {
 	var (
 		wire []byte
@@ -713,3 +736,18 @@ func testRepeatedUInt64Marshal(t *testing.T, n []uint64, wrongField bool) {
 		require.Len(t, result.FieldF, 0)
 	}
 }
+
+func testFixed64Marshal(t *testing.T, n uint64, wrongField bool) {
+	var (
+		custom    = stablePrimitives{FieldI: n}
+		transport = test.Primitives{FieldI: n}
+	)
+
+	result := testMarshal(t, custom, transport, wrongField)
+
+	if !wrongField {
+		require.Equal(t, n, result.FieldI)
+	} else {
+		require.EqualValues(t, 0, result.FieldI)
+	}
+}
diff --git a/util/proto/test/test.pb.go b/util/proto/test/test.pb.go
index 0254baa..a0808f8 100644
Binary files a/util/proto/test/test.pb.go and b/util/proto/test/test.pb.go differ
diff --git a/util/proto/test/test.proto b/util/proto/test/test.proto
index ea6125f..e9c2758 100644
--- a/util/proto/test/test.proto
+++ b/util/proto/test/test.proto
@@ -12,6 +12,7 @@ message Primitives {
     uint32 field_e = 202;
     int64 field_f  = 203;
     uint64 field_g = 204;
+    fixed64 field_i = 205;
 
     enum SomeEnum {
         UNKNOWN = 0;