diff --git a/NanoVNASaver/SITools.py b/NanoVNASaver/SITools.py
index 8d6eaf6..83dff05 100644
--- a/NanoVNASaver/SITools.py
+++ b/NanoVNASaver/SITools.py
@@ -14,6 +14,7 @@
 #
 #  You should have received a copy of the GNU General Public License
 #  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+from __future__ import annotations
 import math
 import decimal
 from typing import NamedTuple, Union