diff --git a/lib/net/scp.rb b/lib/net/scp.rb
index fdbc745..5b04d98 100644
--- a/lib/net/scp.rb
+++ b/lib/net/scp.rb
@@ -360,7 +360,7 @@ def start_command(mode, local, remote, options={}, &callback)
               channel[:buffer  ] = Net::SSH::Buffer.new
               channel[:state   ] = "#{mode}_start"
               channel[:stack   ] = []
-              channel[:error_string] = ''
+              channel[:error_string] = +''
 
               channel.on_close do
                 # If we got an exit-status and it is not 0, something went wrong