diff --git a/include/wlr/render/drm_format_set.h b/include/wlr/render/drm_format_set.h
index 0a6254e2..c8f3c309 100644
--- a/include/wlr/render/drm_format_set.h
+++ b/include/wlr/render/drm_format_set.h
@@ -73,7 +73,8 @@ bool wlr_drm_format_set_add(struct wlr_drm_format_set *set, uint32_t format,
 
 /**
  * Intersect two DRM format sets `a` and `b`, storing in the destination set
- * `dst` the format + modifier pairs which are in both source sets.
+ * `dst` the format + modifier pairs which are in both source sets. The `dst`
+ * must either be zeroed or initialized with other state to be replaced.
  *
  * Returns false on failure or when the intersection is empty.
  */
diff --git a/render/drm_format_set.c b/render/drm_format_set.c
index 464c0fb4..8f3aea99 100644
--- a/render/drm_format_set.c
+++ b/render/drm_format_set.c
@@ -205,8 +205,6 @@ struct wlr_drm_format *wlr_drm_format_intersect(
 
 bool wlr_drm_format_set_intersect(struct wlr_drm_format_set *dst,
 		const struct wlr_drm_format_set *a, const struct wlr_drm_format_set *b) {
-	assert(dst != a && dst != b);
-
 	struct wlr_drm_format_set out = {0};
 	out.capacity = a->len < b->len ? a->len : b->len;
 	out.formats = calloc(out.capacity, sizeof(struct wlr_drm_format *));
@@ -237,6 +235,7 @@ bool wlr_drm_format_set_intersect(struct wlr_drm_format_set *dst,
 		return false;
 	}
 
+	wlr_drm_format_set_finish(dst);
 	*dst = out;
 	return true;
 }