diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index aa6397271b70672f5c693ddd0e7046a4ee835c65..4838fd99466b197ed583f91afc7c647cc3012b7c 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -838,7 +838,7 @@ def normalize(self, **kwargs):
 
         # If there are deps specified but not visited, they're not
         # actually deps of this package.  Raise an error.
-        extra = set(spec_deps.viewkeys()).difference(visited)
+        extra = set(spec_deps.keys()).difference(visited)
 
         # Also subtract out all the packags that provide a needed vpkg
         vdeps = [v for v in self.package.virtual_dependencies()]