diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 05511dd3..f1af1863 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1945,6 +1945,7 @@ def uid_expunge(uid_set)
     # * When +criteria+ is an array, each member is a +SEARCH+ command argument:
     #   * Any SequenceSet sends SequenceSet#valid_string.
     #     These types are converted to SequenceSet for validation and encoding:
+    #     * +Set+
     #     * +Range+
     #     * <tt>-1</tt> translates to <tt>*</tt>
     #     * nested +Array+
@@ -3204,7 +3205,7 @@ def normalize_searching_criteria(criteria)
 
     def coerce_search_arg_to_seqset?(obj)
       case obj
-      when -1          then true
+      when Set, -1     then true
       when Range       then true
       when Array       then true
       else                  false
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index bb498e6b..01c1d5a7 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -1224,6 +1224,9 @@ def test_unselect
       imap.search(["SUBJECT", "1,*"])
       assert_equal 'SUBJECT "1,*"', server.commands.pop.args
 
+      imap.search(["subject", "hello", Set[1, 2, 3, 4, 5, 8, *(10..100)]])
+      assert_equal "subject hello 1:5,8,10:100", server.commands.pop.args
+
       server.on "UID SEARCH", &search_resp
       assert_equal search_result, imap.uid_search(["subject", "hello",
                                                    [1..22, 30..-1]])