diff --git a/native-datatypes.html b/native-datatypes.html
index 73b2371..8a82022 100755
--- a/native-datatypes.html
+++ b/native-datatypes.html
@@ -563,7 +563,7 @@ AttributeError: 'tuple' object has no attribute 'remove'</samp>
 
 <h2 id=sets>Sets</h2>
 
-<p>A <dfn>set</dfn> is an unordered &#8220;bag&#8221; of unique values. A single set can contain values of any datatype. Once you have two sets, you can do standard set operations like union, intersection, and set difference.
+<p>A <dfn>set</dfn> is an unordered &#8220;bag&#8221; of unique values. A single set can contain values of any immutable datatype. Once you have two sets, you can do standard set operations like union, intersection, and set difference.
 
 <h3 id=creating-a-set>Creating A Set</h3>