diff --git a/lib/yui/compressor.rb b/lib/yui/compressor.rb
index 515ccf8..8d706df 100644
--- a/lib/yui/compressor.rb
+++ b/lib/yui/compressor.rb
@@ -98,7 +98,7 @@ def compress(stream_or_string)
           tempfile.close!
         end
 
-        if $?.exitstatus.zero?
+        if $?.exitstatus && $?.exitstatus.zero?
           output
         else
           # Bourne shells tend to blow up here when the command fails, usually