diff --git a/lib/spack/spack/binary_distribution.py b/lib/spack/spack/binary_distribution.py
index 8a333eefe38..8ceeeea7386 100644
--- a/lib/spack/spack/binary_distribution.py
+++ b/lib/spack/spack/binary_distribution.py
@@ -293,10 +293,12 @@ def update_spec(self, spec, found_list):
                         cur_entry["spec"] = new_entry["spec"]
                         break
                 else:
-                    current_list.append = {
-                        "mirror_url": new_entry["mirror_url"],
-                        "spec": new_entry["spec"],
-                    }
+                    current_list.append(
+                        {
+                            "mirror_url": new_entry["mirror_url"],
+                            "spec": new_entry["spec"],
+                        }
+                    )
 
     def update(self, with_cooldown=False):
         """Make sure local cache of buildcache index files is up to date.