diff --git a/fenicsprecice/expression_core.py b/fenicsprecice/expression_core.py
index b8fb29b0..b794a013 100644
--- a/fenicsprecice/expression_core.py
+++ b/fenicsprecice/expression_core.py
@@ -242,7 +242,7 @@ def eval(self, value, x):
         :param x: coordinate where expression has to be evaluated
         :param value: buffer where result has to be returned to
         """
-        assert(MPI.size(MPI.comm_world) > 1)
+        assert(MPI.COMM_WORLD.Get_size() > 1)
         for i in range(self._vals.ndim):
             value[i] = 0