diff --git a/src/escpos/printer/file.py b/src/escpos/printer/file.py
index c2f932c..852d758 100644
--- a/src/escpos/printer/file.py
+++ b/src/escpos/printer/file.py
@@ -80,7 +80,7 @@ class File(Escpos):
                 return
         logging.info("File printer enabled")
 
-    def flush(self):
+    def flush(self) -> None:
         """Flush printing content."""
         self.device.flush()