diff --git a/include/wlr/util/region.h b/include/wlr/util/region.h
index c3f1cbe87..948307a18 100644
--- a/include/wlr/util/region.h
+++ b/include/wlr/util/region.h
@@ -39,8 +39,8 @@ void wlr_region_transform(pixman_region32_t *dst, const pixman_region32_t *src,
 	enum wl_output_transform transform, int width, int height);
 
 /**
- * Expands the region of `distance`. If `distance` is negative, it shrinks the
- * region.
+ * Expands the region by distance on both axis. distance must be
+ * a non-negative number.
  */
 void wlr_region_expand(pixman_region32_t *dst, const pixman_region32_t *src,
 	int distance);
diff --git a/util/region.c b/util/region.c
index a3b13e061..b74c55e54 100644
--- a/util/region.c
+++ b/util/region.c
@@ -111,6 +111,8 @@ void wlr_region_transform(pixman_region32_t *dst, const pixman_region32_t *src,
 
 void wlr_region_expand(pixman_region32_t *dst, const pixman_region32_t *src,
 		int distance) {
+	assert(distance >= 0);
+
 	if (distance == 0) {
 		pixman_region32_copy(dst, src);
 		return;