diff --git a/lib/spack/spack/cmd/url.py b/lib/spack/spack/cmd/url.py
index 38dd6a4aeb0..cb151b26d99 100644
--- a/lib/spack/spack/cmd/url.py
+++ b/lib/spack/spack/cmd/url.py
@@ -339,7 +339,7 @@ def add(self, pkg_name, fetcher):
     for pkg_cls in spack.repo.PATH.all_package_classes():
         npkgs += 1
 
-        for v in pkg_cls.versions:
+        for v in list(pkg_cls.versions):
             try:
                 pkg = pkg_cls(spack.spec.Spec(pkg_cls.name))
                 fetcher = fs.for_package_version(pkg, v)