diff --git a/quantities/quantity.py b/quantities/quantity.py
index 74a425f..8fc355d 100644
--- a/quantities/quantity.py
+++ b/quantities/quantity.py
@@ -629,7 +629,7 @@ def nanargmin(self,axis=None, out=None):
     def nanargmax(self,axis=None, out=None):
         return np.nanargmax(self.magnitude)
 
-    @with_doc(np.ndarray.ptp)
+    @with_doc(np.ptp)
     def ptp(self, axis=None, out=None):
         ret = np.ptp(self.magnitude, axis, None if out is None else out.magnitude)
         dim = self.dimensionality