diff --git a/plotnine/themes/theme_matplotlib.py b/plotnine/themes/theme_matplotlib.py
index 3d474bf72..ac308ed56 100644
--- a/plotnine/themes/theme_matplotlib.py
+++ b/plotnine/themes/theme_matplotlib.py
@@ -136,7 +136,7 @@ def __init__(self, rc=None, fname=None, use_defaults=True):
         )
 
         if use_defaults:
-            self._rcParams.update(mpl.rcParams.copy())
+            self._rcParams.update(mpl.rcParams)
 
         if fname:
             self._rcParams.update(mpl.rc_params_from_file(fname))