diff --git a/lib/spack/spack/environment.py b/lib/spack/spack/environment.py
index 824f60d338..16f745f9e8 100644
--- a/lib/spack/spack/environment.py
+++ b/lib/spack/spack/environment.py
@@ -600,8 +600,8 @@ def _set_user_specs_from_lockfile(self):
         """Copy user_specs from a read-in lockfile."""
         self.spec_lists = {
             user_speclist_name: SpecList(
-                user_speclist_name, [Spec(s)
-                                     for s in self.concretized_user_specs]
+                user_speclist_name,
+                [str(s) for s in self.concretized_user_specs]
             )
         }
 
diff --git a/lib/spack/spack/spec_list.py b/lib/spack/spack/spec_list.py
index 460e7846ed..88b4dd7e96 100644
--- a/lib/spack/spack/spec_list.py
+++ b/lib/spack/spack/spec_list.py
@@ -28,6 +28,12 @@ def __init__(self, name='specs', yaml_list=[], reference={}):
         self.name = name
         self._reference = reference  # TODO: Do we need defensive copy here?
 
+        # Validate yaml_list before assigning
+        if not all(isinstance(s, string_types) or isinstance(s, (list, dict))
+                   for s in yaml_list):
+            raise ValueError(
+                "yaml_list can contain only valid YAML types!  Found:\n  %s"
+                % [type(s) for s in yaml_list])
         self.yaml_list = yaml_list[:]
 
         # Expansions can be expensive to compute and difficult to keep updated