diff --git a/lib/spack/spack/repository.py b/lib/spack/spack/repository.py
index 63ae999ce1b..6aa9b8dd2f0 100644
--- a/lib/spack/spack/repository.py
+++ b/lib/spack/spack/repository.py
@@ -864,6 +864,12 @@ def get_pkg_class(self, pkg_name):
         package. Then extracts the package class from the module
         according to Spack's naming convention.
         """
+        fullname = pkg_name
+        namespace, _, pkg_name = pkg_name.rpartition('.')
+        if namespace and (namespace != self.namespace):
+            raise InvalidNamespaceError('Invalid namespace for %s repo: %s'
+                                        % (self.namespace, namespace))
+
         class_name = mod_to_class(pkg_name)
         module = self._get_pkg_module(pkg_name)
 
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index b9d9d3e0a46..24459fd3b3f 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -714,7 +714,7 @@ def package_class(self):
         """Internal package call gets only the class object for a package.
            Use this to just get package metadata.
         """
-        return spack.repo.get_pkg_class(self.name)
+        return spack.repo.get_pkg_class(self.fullname)
 
     @property
     def virtual(self):
@@ -1574,7 +1574,7 @@ def validate_names(self):
            UnsupportedCompilerError.
         """
         for spec in self.traverse():
-            # Don't get a package for a virtual name.
+            # raise an UnknownPackageError if the spec's package isn't real.
             if (not spec.virtual) and spec.name:
                 spack.repo.get(spec.fullname)