diff --git a/sho/num.py b/sho/num.py
index 8439872..bb5e86c 100644
--- a/sho/num.py
+++ b/sho/num.py
@@ -17,7 +17,7 @@ def to_sensors(sol):
     assert(len(sol)>0)
     sensors = []
     for i in range(0,len(sol),2):
-        sensors.append( ( int(floor(sol[i])), int(floor(sol[i+1])) ) )
+        sensors.append( ( int(math.floor(sol[i])), int(math.floor(sol[i+1])) ) )
     return sensors