diff --git a/lib/spack/spack/preferred_packages.py b/lib/spack/spack/preferred_packages.py
index 5f18e212b6c..f079c1ef8b0 100644
--- a/lib/spack/spack/preferred_packages.py
+++ b/lib/spack/spack/preferred_packages.py
@@ -162,8 +162,8 @@ def spec_preferred_variants(self, pkgname):
         """Return a VariantMap of preferred variants and their values"""
         variants = self.preferred.get(pkgname, {}).get('variants', '')
         if not isinstance(variants, basestring):
-            variants = "".join(variants)
-        return spack.spec.Spec(pkgname + variants).variants
+            variants = " ".join(variants)
+        return spack.spec.Spec("%s %s" % (pkgname, variants)).variants
 
     def version_compare(self, pkgname, a, b):
         """Return less-than-0, 0, or greater than 0 if version a of pkgname is