diff --git a/lib/spack/spack/test/mock_packages_test.py b/lib/spack/spack/test/mock_packages_test.py
index 23b1cf52cc..5561027aac 100644
--- a/lib/spack/spack/test/mock_packages_test.py
+++ b/lib/spack/spack/test/mock_packages_test.py
@@ -137,6 +137,10 @@
     paths:
       externalvirtual@2.0%clang@3.3: /path/to/external_virtual_clang
       externalvirtual@1.0%gcc@4.5.0: /path/to/external_virtual_gcc
+  externalmodule:
+    buildable: False
+    modules:
+      externalmodule@1.0%gcc@4.5.0: external-module
 """
 
 class MockPackagesTest(unittest.TestCase):