diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index f13ff1a..c3fe0b4 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -34,7 +34,7 @@ def self.parser=(parser = RFC3986_PARSER)
     end
 
     Parser.new.regexp.each_pair do |sym, str|
-      remove_const(sym) if const_defined?(sym)
+      remove_const(sym) if const_defined?(sym, false)
       const_set(sym, str)
     end
   end
diff --git a/lib/uri/rfc2396_parser.rb b/lib/uri/rfc2396_parser.rb
index c112b1b..0336366 100644
--- a/lib/uri/rfc2396_parser.rb
+++ b/lib/uri/rfc2396_parser.rb
@@ -539,7 +539,7 @@ def convert_to_uri(uri)
 
   # Backward compatibility for URI::REGEXP::PATTERN::*
   RFC2396_Parser.new.pattern.each_pair do |sym, str|
-    unless RFC2396_REGEXP::PATTERN.const_defined?(sym)
+    unless RFC2396_REGEXP::PATTERN.const_defined?(sym, false)
       RFC2396_REGEXP::PATTERN.const_set(sym, str)
     end
   end