diff --git a/lib/spack/spack/compilers/__init__.py b/lib/spack/spack/compilers/__init__.py
index 6731318f43..87106282cf 100644
--- a/lib/spack/spack/compilers/__init__.py
+++ b/lib/spack/spack/compilers/__init__.py
@@ -216,9 +216,9 @@ def compiler_for_spec(compiler_spec, target):
     assert(compiler_spec.concrete)
     compilers = compilers_for_spec(compiler_spec)
     if target.compiler_strategy == "PATH":
-        filter(lambda c: c.modules is None, compilers)
+        compilers = [c for c in compilers if c.modules is None]
     elif target.compiler_strategy == "MODULES":
-        filter(lambda c: c.modules is not None, compilers)
+        compilers = [c for c in compilers if c.modules is not None]
     assert(len(compilers) == 1)
     return compilers[0]
 
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index d66ffb338e..6e11cfa475 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -277,7 +277,7 @@ def is_spec_nobuild(spec):
 
 
 def spec_externals(spec):
-    """Return a list of spec, directory pairs for each external location for spec"""
+    """Return a list of spec, directory, module triples for each external location for spec"""
     allpkgs = get_packages_config()
     name = spec.name
     spec_locations = []
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index f496011d62..126ffc4f95 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -771,8 +771,9 @@ def _replace_with(self, concrete):
         """Replace this virtual spec with a concrete spec."""
         assert(self.virtual)
         for name, dependent in self.dependents.items():
-            del dependent.dependencies[self.name]
-            dependent._add_dependency(concrete)
+            if not dependent.external:
+                del dependent.dependencies[self.name]
+                dependent._add_dependency(concrete)
 
 
     def _expand_virtual_packages(self):