diff --git a/adafruit_us100.py b/adafruit_us100.py
index 0ee2f1a..9a8adee 100644
--- a/adafruit_us100.py
+++ b/adafruit_us100.py
@@ -25,15 +25,21 @@
 
 import time
 
+try:
+    from typing import Optional
+    from busio import UART
+except ImportError:
+    pass
+
 
 class US100:
     """Control a US-100 ultrasonic range sensor."""
 
-    def __init__(self, uart):
+    def __init__(self, uart: UART) -> None:
         self._uart = uart
 
     @property
-    def distance(self):
+    def distance(self) -> Optional[float]:
         """Return the distance measured by the sensor in cm.
         This is the function that will be called most often in user code.
         If no signal is received, return ``None``. This can happen when the
@@ -45,7 +51,7 @@ def distance(self):
         for the sensor to handle. In my experience, the sensor can not detect
         objects over 460 cm away.
         :return: Distance in centimeters.
-        :rtype: float
+        :rtype: float or None
         """
         for _ in range(2):  # Attempt to read twice.
             self._uart.write(bytes([0x55]))
@@ -67,7 +73,7 @@ def distance(self):
         return dist
 
     @property
-    def temperature(self):
+    def temperature(self) -> Optional[int]:
         """Return the on-chip temperature, in Celsius"""
         for _ in range(2):  # Attempt to read twice.
             self._uart.write(bytes([0x50]))