diff --git a/lib/irb/completion.rb b/lib/irb/completion.rb
index 10533c5433..7277bf5504 100644
--- a/lib/irb/completion.rb
+++ b/lib/irb/completion.rb
@@ -101,6 +101,8 @@ module IRB
           sym = $1
           candidates = Symbol.all_symbols.collect do |s|
             ":" + s.id2name.encode(Encoding.default_external)
+          rescue Encoding::UndefinedConversionError
+            # ignore
           end
           candidates.grep(/^#{Regexp.quote(sym)}/)
         else