diff --git a/src/calibre/trac/plugins/download.py b/src/calibre/trac/plugins/download.py
index 1a04a6f6aa..6436aba448 100644
--- a/src/calibre/trac/plugins/download.py
+++ b/src/calibre/trac/plugins/download.py
@@ -429,6 +429,6 @@ def main():
     print 'Extracting files to %s ...'%destdir
     extract_tarball(f, destdir)
     pi = os.path.join(destdir, 'calibre_postinstall')
-    subprocess.call('pi', shell=True)
+    subprocess.call(pi, shell=True)
     return 0
 '''