diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 9c9e9e10ff5de9c08c9fada53511df1c7cbc0cf8..2351e2bfc9eea9b554d0cc13ac08b9663d495f46 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -95,7 +95,11 @@ def cmp_externals(a, b):
                                      not b.external and b.external_module):
                 # We're choosing between different providers, so
                 # maintain order from provider sort
-                return candidates.index(a) - candidates.index(b)
+                index_of_a = next(i for i in range(0, len(candidates))
+                                  if a.satisfies(candidates[i]))
+                index_of_b = next(i for i in range(0, len(candidates))
+                                  if b.satisfies(candidates[i]))
+                return index_of_a - index_of_b
 
             result = cmp_specs(a, b)
             if result != 0:
diff --git a/var/spack/repos/builtin.mock/packages/externalvirtual/package.py b/var/spack/repos/builtin.mock/packages/externalvirtual/package.py
index 1f3553014b2722c3de12f07b6d493f7974715d47..6310a17bc94f49a14054fec0724c7593a507bc23 100644
--- a/var/spack/repos/builtin.mock/packages/externalvirtual/package.py
+++ b/var/spack/repos/builtin.mock/packages/externalvirtual/package.py
@@ -31,8 +31,10 @@ class Externalvirtual(Package):
 
     version('1.0', '1234567890abcdef1234567890abcdef')
     version('2.0', '234567890abcdef1234567890abcdef1')
+    version('2.1', '34567890abcdef1234567890abcdef12')
+    version('2.2', '4567890abcdef1234567890abcdef123')
 
-    provides('stuff')
+    provides('stuff', when='@1.0:')
 
     def install(self, spec, prefix):
         pass
diff --git a/var/spack/repos/builtin.mock/packages/othervirtual/package.py b/var/spack/repos/builtin.mock/packages/othervirtual/package.py
new file mode 100644
index 0000000000000000000000000000000000000000..83bc07df98abefb6efefc1fcd949963cda1d6f3b
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/othervirtual/package.py
@@ -0,0 +1,37 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class Othervirtual(Package):
+    homepage = "http://somewhere.com"
+    url      = "http://somewhere.com/stuff-1.0.tar.gz"
+
+    version('1.0', '67890abcdef1234567890abcdef12345')
+
+    provides('stuff')
+
+    def install(self, spec, prefix):
+        pass