diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 0752908a389..a65b5fc60f1 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1374,8 +1374,13 @@ def cshort_spec(self):
 
     @property
     def prefix(self):
+        if hasattr(self, 'test_prefix'):
+            return Prefix(self.test_prefix)
         return Prefix(spack.store.layout.path_for_spec(self))
 
+    def _set_test_prefix(self, val):
+        self.test_prefix = val
+
     def dag_hash(self, length=None):
         """Return a hash of the entire spec DAG, including connectivity."""
         if self._hash: