diff --git a/qsdsan/sanunits/_abstract.py b/qsdsan/sanunits/_abstract.py
index 8a99237e..0b29272e 100644
--- a/qsdsan/sanunits/_abstract.py
+++ b/qsdsan/sanunits/_abstract.py
@@ -58,10 +58,11 @@ class Mixer(SanUnit, BSTMixer):
     _graphics = BSTMixer._graphics
     def __init__(self, ID='', ins=None, outs=(), thermo=None,
                  init_with='WasteStream', F_BM_default=None, isdynamic=False,
-                 rigorous=False):
+                 rigorous=False, conserve_phases=False):
         SanUnit.__init__(self, ID, ins, outs, thermo, init_with,
                          F_BM_default=F_BM_default, isdynamic=isdynamic)
         self.rigorous = rigorous
+        self.conserve_phases = conserve_phases
 
 
     @property