diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index cb3e41a3..5f10a187 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -1342,8 +1342,8 @@ def tuple_add(tuple)
         modifying!
         min, max = tuple
         lower, lower_idx = tuple_gte_with_index(min - 1)
-        if    lower.nil?              then tuples << tuple
-        elsif (max + 1) < lower.first then tuples.insert(lower_idx, tuple)
+        if    lower.nil?              then tuples << [min, max]
+        elsif (max + 1) < lower.first then tuples.insert(lower_idx, [min, max])
         else  tuple_coalesce(lower, lower_idx, min, max)
         end
       end
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index e2f1ce7e..fa91288e 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -359,6 +359,11 @@ def obj.to_sequence_set; 192_168.001_255 end
     assert_equal seqset["1:3,5,7:9"], seqset["1,3,5,7:8"].merge(seqset["2,8:9"])
     assert_equal seqset["1:*"],       seqset["5:*"].merge(1..4)
     assert_equal seqset["1:5"],       seqset["1,3,5"].merge(seqset["2,4"])
+    # when merging frozen SequenceSet
+    set = SequenceSet.new
+    set.merge SequenceSet[1, 3, 5]
+    set.merge SequenceSet[2..33]
+    assert_equal seqset[1..33], set
   end
 
   test "set - other" do