diff --git a/lib/ssh_scan/scan_engine.rb b/lib/ssh_scan/scan_engine.rb
index 631be1b2..7526e10f 100644
--- a/lib/ssh_scan/scan_engine.rb
+++ b/lib/ssh_scan/scan_engine.rb
@@ -126,7 +126,9 @@ def scan_target(socket, opts)
       cmd = ['ssh-keyscan', '-t', 'rsa,dsa', '-p', port.to_s, target].join(" ")
 
       Utils::Subprocess.new(cmd) do |stdout, stderr, thread|
-        output += stdout
+        if stdout
+          output += stdout
+        end
       end
 
       host_keys = output.split