diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index a5dabfc7..10ceec39 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -1922,7 +1922,7 @@ def input_try_convert(input)
       def number_input?(input)
         case input
         when *STARS, Integer then true
-        when String          then !input.include?(/[:,]/)
+        when String          then !input.match?(/[:,]/)
         end
       end
 
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index fd6efa12..1e98e875 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -858,6 +858,7 @@ def obj.to_sequence_set; 192_168.001_255 end
     set = SequenceSet.new [5..11, 20, 30..40]
     # deleting a single-member range
     assert_equal SequenceSet[9], set.delete?(9..9)
+    assert_equal 6, SequenceSet.new(5..10).delete?("6")
   end
 
   test "#slice!" do