diff --git a/benchmarks/benchmark_PhysicsSuite.py b/benchmarks/benchmark_PhysicsSuite.py
index 6fa47ae..f2192fe 100644
--- a/benchmarks/benchmark_PhysicsSuite.py
+++ b/benchmarks/benchmark_PhysicsSuite.py
@@ -201,7 +201,7 @@ def setup(self):
         pass
 
     def time_mass_density(self):
-        mass_density(1 * u.m ** -3)
+        mass_density(1 * u.m ** -3,'p')
 
 
     def time_Alfven_speed(self):