diff --git a/tudplot/utils.py b/tudplot/utils.py
index 751e6e2..11a022f 100644
--- a/tudplot/utils.py
+++ b/tudplot/utils.py
@@ -33,10 +33,10 @@ def facet_plot(dframe, facets, props, ydata, layout=None, newfig=True, figsize=N
     cycl = cycle(plt.rcParams['axes.prop_cycle'])
     prop_styles = {ps: next(cycl) for ps, _ in dframe.groupby(props)}
 
-    if zorder is 'default':
+    if zorder == 'default':
         dz = 1
         zorder = 0
-    elif zorder is 'reverse':
+    elif zorder == 'reverse':
         dz = -1
         zorder = 0
     else: