diff --git a/src/themes.jl b/src/themes.jl
index aed8ecc0..1dc102d9 100644
--- a/src/themes.jl
+++ b/src/themes.jl
@@ -4,7 +4,7 @@
 Specify the colour theme for plots.
 """
 function theme(s::Symbol; kw...)
-    defaults = PlotThemes._themes[s].defaults
+    defaults = copy(PlotThemes._themes[s].defaults)
     _theme(s, defaults; kw...)
 end