diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index 1cb85112..b046b0eb 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -684,8 +684,9 @@ def append(object)
         modifying!
         tuple = input_to_tuple object
         entry = tuple_to_str tuple
+        string unless empty? # write @string before tuple_add
         tuple_add tuple
-        @string = -(string ? "#{@string},#{entry}" : entry)
+        @string = -(@string ? "#{@string},#{entry}" : entry)
         self
       end
 
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index 85204b24..94c096ba 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -344,6 +344,14 @@ def obj.to_sequence_set; 192_168.001_255 end
     assert_equal "1:6,4:9", SequenceSet.new("1:6").append("4:9").string
     assert_equal "1:4,5:*", SequenceSet.new("1:4").append(5..).string
     assert_equal "5:*,1:4", SequenceSet.new("5:*").append(1..4).string
+    # also works from empty
+    assert_equal "5,1",     SequenceSet.new.append(5).append(1).string
+    # also works when *previously* input was non-strings
+    assert_equal "*,1",     SequenceSet.new(:*).append(1).string
+    assert_equal "1,5",     SequenceSet.new(1).append("5").string
+    assert_equal "1:6,4:9", SequenceSet.new(1..6).append(4..9).string
+    assert_equal "1:4,5:*", SequenceSet.new(1..4).append(5..).string
+    assert_equal "5:*,1:4", SequenceSet.new(5..).append(1..4).string
   end
 
   test "#merge" do