diff --git a/hdbscan/_hdbscan_tree.pyx b/hdbscan/_hdbscan_tree.pyx
index dacce614..b4f07e71 100644
--- a/hdbscan/_hdbscan_tree.pyx
+++ b/hdbscan/_hdbscan_tree.pyx
@@ -679,12 +679,12 @@ cpdef np.ndarray simplify_hierarchy(np.ndarray condensed_tree,
     cdef np.ndarray cluster_tree = condensed_tree[condensed_tree['child_size'] > 1]
     cdef np.ndarray cluster_mask, keep_mask, update_mask
     keep_mask = np.ones(condensed_tree.shape[0], dtype=np.bool_)
+    processed = set()
     while cluster_tree.shape[0] > 0:
         leaves = set(get_cluster_tree_leaves(cluster_tree))
         births = max_lambdas(condensed_tree)
         deaths = min_lambdas(cluster_tree, leaves)
         
-        processed = set()
         cluster_mask = np.ones(cluster_tree.shape[0], dtype=np.bool_)
         for leaf in sorted(leaves, reverse=True):
             if leaf in processed or (births[leaf] - deaths[leaf]) >= persistence_threshold: