diff --git a/include/tclap/ValuesConstraint.h b/include/tclap/ValuesConstraint.h
index a234297..c9ca80a 100644
--- a/include/tclap/ValuesConstraint.h
+++ b/include/tclap/ValuesConstraint.h
@@ -50,7 +50,7 @@ class ValuesConstraint : public Constraint<T>
 		 * Constructor. 
 		 * \param allowed - vector of allowed values. 
 		 */
-		ValuesConstraint(std::vector<T>& allowed);	
+		ValuesConstraint(std::vector<T>const& allowed);
 
 		/**
 		 * Virtual destructor.
@@ -89,7 +89,7 @@ class ValuesConstraint : public Constraint<T>
 };
 
 template<class T>
-ValuesConstraint<T>::ValuesConstraint(std::vector<T>& allowed)
+ValuesConstraint<T>::ValuesConstraint(std::vector<T> const& allowed)
 : _allowed(allowed),
   _typeDesc("")
 {