diff --git a/lib/oga/xpath/conversion.rb b/lib/oga/xpath/conversion.rb
index 30bd938..e9b6b27 100644
--- a/lib/oga/xpath/conversion.rb
+++ b/lib/oga/xpath/conversion.rb
@@ -89,7 +89,7 @@ module Oga
 
       # @return [TrueClass|FalseClass]
       def self.boolean?(value)
-        value.is_a?(TrueClass) || value.is_a?(FalseClass)
+        value == true || value == false
       end
 
       # @param [Oga::XML::NodeSet] set