diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 1e2435eac2..eca984093d 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -381,7 +381,7 @@ def _execute_extends(pkg):
         #     msg = 'Packages can extend at most one other package.'
         #     raise DirectiveError(directive, msg)
 
-        when = kwargs.pop('when', pkg.name)
+        when = kwargs.get('when', pkg.name)
         _depends_on(pkg, spec, when=when)
         pkg.extendees[spec] = (Spec(spec), kwargs)
     return _execute_extends