diff --git a/openmc/material.py b/openmc/material.py
index 87cc6befaa..202f46c1ba 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -380,7 +380,7 @@ class Material(object):
         cv.check_value('density units', units, DENSITY_UNITS)
         self._density_units = units
 
-        if units is 'sum':
+        if units == 'sum':
             if density is not None:
                 msg = 'Density "{}" for Material ID="{}" is ignored ' \
                       'because the unit is "sum"'.format(density, self.id)
diff --git a/openmc/model/triso.py b/openmc/model/triso.py
index 046bd45159..2090dc977d 100644
--- a/openmc/model/triso.py
+++ b/openmc/model/triso.py
@@ -970,13 +970,13 @@ def pack_trisos(radius, fill, domain_shape='cylinder', domain_length=None,
         raise ValueError('"domain_radius" must be specified for {} domain '
                          'geometry '.format(domain_shape))
 
-    if domain_shape is 'cube':
+    if domain_shape == 'cube':
         domain = _CubicDomain(length=domain_length, particle_radius=radius,
                               center=domain_center)
-    elif domain_shape is 'cylinder':
+    elif domain_shape == 'cylinder':
         domain = _CylindricalDomain(length=domain_length, radius=domain_radius,
                                     particle_radius=radius, center=domain_center)
-    elif domain_shape is 'sphere':
+    elif domain_shape == 'sphere':
         domain = _SphericalDomain(radius=domain_radius, particle_radius=radius,
                                   center=domain_center)