diff --git a/lib/edn/core_ext.rb b/lib/edn/core_ext.rb
index 825dfdb..046cf6b 100644
--- a/lib/edn/core_ext.rb
+++ b/lib/edn/core_ext.rb
@@ -101,7 +101,7 @@ def to_edn
 
 Numeric.send(:include, EDN::CoreExt::Unquoted)
 Integer.send(:include, EDN::CoreExt::Integer)
-Bignum.send(:include, EDN::CoreExt::Bignum)
+Bignum.send(:include, EDN::CoreExt::Bignum) if defined?(Bignum)
 BigDecimal.send(:include, EDN::CoreExt::BigDecimal)
 TrueClass.send(:include, EDN::CoreExt::Unquoted)
 FalseClass.send(:include, EDN::CoreExt::Unquoted)