diff --git a/mpcmd/base.py b/mpcmd/base.py
index 8f810fe..50b60c3 100644
--- a/mpcmd/base.py
+++ b/mpcmd/base.py
@@ -84,7 +84,7 @@ def create_fluid(self, geometry=None, position=None, velocity=None, N=0, density
                 velocity = None
                 
         if velocity is None:
-            scales = np.sqrt(KbT/arr([mass])).T
+            scales = np.sqrt(KbT/mass)
             velocity = scales*np.random.randn(self.N,3)
             
         self.position = position