diff --git a/entry.c b/entry.c
index 7b9f43716f..891e4ba2b4 100644
--- a/entry.c
+++ b/entry.c
@@ -412,6 +412,8 @@ static void mark_colliding_entries(const struct checkout *state,
 
 	ce->ce_flags |= CE_MATCHED;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(state->istate);
 	for (i = 0; i < state->istate->cache_nr; i++) {
 		struct cache_entry *dup = state->istate->cache[i];