diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index bf64efa3ec5..13bcfd2e915 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -1028,6 +1028,10 @@ def external_packages(self):
             if pkg_name == 'all':
                 continue
 
+            # This package does not appear in any repository
+            if pkg_name not in spack.repo.path:
+                continue
+
             if 'externals' not in data:
                 self.gen.fact(fn.external(pkg_name).symbol(positive=False))
 
diff --git a/lib/spack/spack/util/mock_package.py b/lib/spack/spack/util/mock_package.py
index 4751f5af7e4..5286b50464d 100644
--- a/lib/spack/spack/util/mock_package.py
+++ b/lib/spack/spack/util/mock_package.py
@@ -102,6 +102,9 @@ def repo_for_pkg(self, name):
         Repo = collections.namedtuple('Repo', ['namespace'])
         return Repo('mockrepo')
 
+    def __contains__(self, item):
+        return item in self.spec_to_pkg
+
     def add_package(self, name, dependencies=None, dependency_types=None,
                     conditions=None):
         """Factory method for creating mock packages.