diff --git a/thinkbayes2/thinkbayes2.py b/thinkbayes2/thinkbayes2.py
index a9e48c93..fe3ff099 100644
--- a/thinkbayes2/thinkbayes2.py
+++ b/thinkbayes2/thinkbayes2.py
@@ -1223,7 +1223,7 @@ def PercentileRanks(self, xs):
 
         returns: array of percentile ranks in the range 0 to 100
         """
-        return self.Probs(x) * 100
+        return self.Probs(xs) * 100
 
     def Random(self):
         """Chooses a random value from this distribution."""