diff --git a/var/spack/repos/builtin/packages/xqilla/package.py b/var/spack/repos/builtin/packages/xqilla/package.py
index 30970706046..d187b0a09cd 100644
--- a/var/spack/repos/builtin/packages/xqilla/package.py
+++ b/var/spack/repos/builtin/packages/xqilla/package.py
@@ -51,6 +51,6 @@ def configure_args(self):
                          '--with-pic'])
 
         if '+debug' in self.spec:
-            args.extend('--enable-debug')
+            args.append('--enable-debug')
 
         return args