diff --git a/lib/irb/completion.rb b/lib/irb/completion.rb
index 64e387d49c9f27..a143d1b3e112e1 100644
--- a/lib/irb/completion.rb
+++ b/lib/irb/completion.rb
@@ -218,7 +218,7 @@ def self.retrieve_completion_data(input, bind: IRB.conf[:MAIN_CONTEXT].workspace
         else
           sym = $1
           candidates = Symbol.all_symbols.collect do |s|
-            ":" + s.id2name.encode(Encoding.default_external)
+            s.inspect
           rescue EncodingError
             # ignore
           end
diff --git a/test/irb/test_completion.rb b/test/irb/test_completion.rb
index 6a24f0f9bab496..8cd14fed52b351 100644
--- a/test/irb/test_completion.rb
+++ b/test/irb/test_completion.rb
@@ -289,12 +289,14 @@ def has_rdoc_content?
     end
 
     def test_complete_symbol
-      %w"UTF-16LE UTF-7".each do |enc|
+      symbols = %w"UTF-16LE UTF-7".map do |enc|
         "K".force_encoding(enc).to_sym
       rescue
       end
-      _ = :aiueo
-      assert_include(IRB::InputCompletor.retrieve_completion_data(":a", bind: binding), ":aiueo")
+      symbols += [:aiueo, :"aiu eo"]
+      candidates = IRB::InputCompletor.retrieve_completion_data(":a", bind: binding)
+      assert_include(candidates, ":aiueo")
+      assert_not_include(candidates, ":aiu eo")
       assert_empty(IRB::InputCompletor.retrieve_completion_data(":irb_unknown_symbol_abcdefg", bind: binding))
       # Do not complete empty symbol for performance reason
       assert_empty(IRB::InputCompletor.retrieve_completion_data(":", bind: binding))