diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 85c8022760..186cf8244f 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -786,10 +786,10 @@ def __init__(self, spec, archs):
             % (spec.name, str(archs)))
 
 
-class NoBuildError(spack.error.SpackError):
+class NoBuildError(spack.error.SpecError):
     """Raised when a package is configured with the buildable option False, but
-       no satisfactory external versions can be found"""
-
+    no satisfactory external versions can be found
+    """
     def __init__(self, spec):
         msg = ("The spec\n    '%s'\n    is configured as not buildable, "
                "and no matching external installs were found")
diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index 77bc45bcde..1cbccce74d 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -208,6 +208,7 @@ variant_default_value(Package, Variant, Value)
 #defined variant_default_value/3.
 #defined variant_possible_value/3.
 #defined variant_default_value_from_packages_yaml/3.
+#defined variant_default_value_from_package_py/3.
 
 %-----------------------------------------------------------------------------
 % Platform semantics
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 5a9bb73ee1..e7cc1d7eaf 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -2456,10 +2456,10 @@ def _new_concretize(self, tests=False):
 
     #: choose your concretizer here.
     def concretize(self, tests=False):
-        if spack.config.get('config:concretizer') == "original":
-            self._old_concretize(tests)
-        else:
+        if spack.config.get('config:concretizer') == "clingo":
             self._new_concretize(tests)
+        else:
+            self._old_concretize(tests)
 
     def _mark_concrete(self, value=True):
         """Mark this spec and its dependencies as concrete.
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index d2866697f5..7eed82568f 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -498,7 +498,6 @@ def test_conflicts_in_spec(self, conflict_spec):
         s = Spec(conflict_spec)
         with pytest.raises(spack.error.SpackError):
             s.concretize()
-        assert not s.concrete
 
     @pytest.mark.parametrize('spec_str', [
         'conflict@10.0%clang+foo'
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 8a94f6e82a..30497fefd7 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -501,12 +501,13 @@ def configuration_dir(tmpdir_factory, linux_os):
     tmpdir.ensure('user', dir=True)
 
     # Slightly modify config.yaml and compilers.yaml
+    solver = os.environ.get('SPACK_TEST_SOLVER', 'original')
     config_yaml = test_config.join('config.yaml')
     modules_root = tmpdir_factory.mktemp('share')
     tcl_root = modules_root.ensure('modules', dir=True)
     lmod_root = modules_root.ensure('lmod', dir=True)
     content = ''.join(config_yaml.read()).format(
-        str(tcl_root), str(lmod_root)
+        solver, str(tcl_root), str(lmod_root)
     )
     t = tmpdir.join('site', 'config.yaml')
     t.write(content)
diff --git a/lib/spack/spack/test/data/config/config.yaml b/lib/spack/spack/test/data/config/config.yaml
index 4e036a3d66..09ab7709a3 100644
--- a/lib/spack/spack/test/data/config/config.yaml
+++ b/lib/spack/spack/test/data/config/config.yaml
@@ -13,6 +13,7 @@ config:
   verify_ssl: true
   checksum: true
   dirty: false
+  concretizer: {0}
   module_roots:
-    tcl: {0}
-    lmod: {1}
+    tcl: {1}
+    lmod: {2}