diff --git a/include/wlr/types/wlr_surface.h b/include/wlr/types/wlr_surface.h
index b6d5f79f..38d6b453 100644
--- a/include/wlr/types/wlr_surface.h
+++ b/include/wlr/types/wlr_surface.h
@@ -114,4 +114,9 @@ int wlr_surface_set_role(struct wlr_surface *surface, const char *role,
 void wlr_surface_make_subsurface(struct wlr_surface *surface,
 		struct wlr_surface *parent, uint32_t id);
 
+/**
+ * Get the top of the subsurface tree for this surface.
+ */
+struct wlr_surface *wlr_surface_get_main_surface(struct wlr_surface *surface);
+
 #endif
diff --git a/types/wlr_compositor.c b/types/wlr_compositor.c
index a142ddd4..12903cda 100644
--- a/types/wlr_compositor.c
+++ b/types/wlr_compositor.c
@@ -84,10 +84,31 @@ static void subcompositor_get_subsurface(struct wl_client *client,
 	struct wlr_surface *surface = wl_resource_get_user_data(surface_resource);
 	struct wlr_surface *parent = wl_resource_get_user_data(parent_resource);
 
-	// TODO: errors
-	// * cannot be its own parent
-	// * cannot already a subsurface
-	// * cannot be an ancestor of parent
+	static const char msg[] = "get_subsurface: wl_subsurface@";
+
+	if (surface == parent) {
+		wl_resource_post_error(resource,
+			WL_SUBCOMPOSITOR_ERROR_BAD_SURFACE,
+			"%s%d: wl_surface@%d cannot be its own parent",
+			msg, id, wl_resource_get_id(surface_resource));
+		return;
+	}
+
+	if (surface->subsurface) {
+		wl_resource_post_error(resource,
+			WL_SUBCOMPOSITOR_ERROR_BAD_SURFACE,
+			"%s%d: wl_surface@%d is already a sub-surface",
+			msg, id, wl_resource_get_id(surface_resource));
+		return;
+	}
+
+	if (wlr_surface_get_main_surface(parent) == surface) {
+		wl_resource_post_error(resource,
+			WL_SUBCOMPOSITOR_ERROR_BAD_SURFACE,
+			"%s%d: wl_surface@%d is an ancestor of parent",
+			msg, id, wl_resource_get_id(surface_resource));
+		return;
+	}
 
 	if (wlr_surface_set_role(surface, "wl_subsurface", resource,
 				WL_SUBCOMPOSITOR_ERROR_BAD_SURFACE) < 0) {
diff --git a/types/wlr_surface.c b/types/wlr_surface.c
index 6f6df226..8c084ceb 100644
--- a/types/wlr_surface.c
+++ b/types/wlr_surface.c
@@ -769,3 +769,14 @@ void wlr_surface_make_subsurface(struct wlr_surface *surface,
 
 	surface->subsurface = subsurface;
 }
+
+
+struct wlr_surface *wlr_surface_get_main_surface(struct wlr_surface *surface) {
+	struct wlr_subsurface *sub;
+
+	while (surface && (sub = surface->subsurface)) {
+		surface = sub->parent;
+	}
+
+	return surface;
+}