diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index 971a684..813099a 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -54,7 +54,7 @@ def self.const_missing(const) # :nodoc:
     elsif value = RFC2396_PARSER.regexp[const]
       warn "URI::#{const} is obsolete. Use URI::RFC2396_PARSER.regexp[#{const.inspect}] explicitly.", uplevel: 1 if $VERBOSE
       value
-    elsif value = RFC2396_Parser.const_get(const)
+    elsif RFC2396_Parser.const_defined?(const) && value = RFC2396_Parser.const_get(const)
       warn "URI::#{const} is obsolete. Use URI::RFC2396_Parser::#{const} explicitly.", uplevel: 1 if $VERBOSE
       value
     else
diff --git a/test/uri/test_common.rb b/test/uri/test_common.rb
index 5692640..21643e4 100644
--- a/test/uri/test_common.rb
+++ b/test/uri/test_common.rb
@@ -19,7 +19,8 @@ class Foo
 
   def test_fallback_constants
     EnvUtil.suppress_warning do
-      assert_raise(NameError) { URI::FOO }
+      e = assert_raise(NameError) { URI::FOO }
+      assert_equal(e.message, "uninitialized constant URI::FOO")
 
       assert_equal URI::ABS_URI, URI::RFC2396_PARSER.regexp[:ABS_URI]
       assert_equal URI::PATTERN, URI::RFC2396_Parser::PATTERN