diff --git a/py/hexagram/thermo.py b/py/hexagram/thermo.py
index 4344351..be2d450 100644
--- a/py/hexagram/thermo.py
+++ b/py/hexagram/thermo.py
@@ -36,7 +36,7 @@ class Thermometer(BottomDial):
         cr.set_line_width(6.0)
 
         level = self.min_value
-        incr  = math.ceil((self.max_value - self.min_value) / 9)
+        incr  = math.ceil((self.max_value - self.min_value) / 9.0)
 
         while level <= self.max_value:
             if level >= self.WARN_VALUE: