diff --git a/lib/irb/cmd/show_source.rb b/lib/irb/cmd/show_source.rb
index dcba1d1c71..feff8315f6 100644
--- a/lib/irb/cmd/show_source.rb
+++ b/lib/irb/cmd/show_source.rb
@@ -61,12 +61,15 @@ module IRB
         lex = RubyLex.new
         lines = File.read(file).lines[(first_line - 1)..-1]
         tokens = RubyLex.ripper_lex_without_warning(lines.join)
+
+        code = +""
         prev_tokens = []
 
         # chunk with line number
         tokens.chunk { |tok| tok[0][0] }.each do |lnum, chunk|
-          code = lines[0..lnum].join
+          code << lines[lnum]
           prev_tokens.concat chunk
+
           continue = lex.process_continue(prev_tokens)
           code_block_open = lex.check_code_block(code, prev_tokens)
           if !continue && !code_block_open