diff --git a/cameo/path.c b/cameo/path.c
index c2a4707..f8a8515 100644
--- a/cameo/path.c
+++ b/cameo/path.c
@@ -106,6 +106,17 @@ static int path_is_closed(const struct path *path)
 }
 
 
+static void assert_path_is_closed(const struct path *path)
+{
+	if (path_is_closed(path))
+		return;
+	fprintf(stderr, "path from (%g, %g, %g) to (%g, %g, %g) is open\n",
+	    path->first->x, path->first->y, path->first->z,
+	    path->last->x, path->last->y, path->last->z);
+	abort();
+}
+
+
 static void path_add_point(struct path *path, struct point *p)
 {
 	if (path->last &&
@@ -284,7 +295,8 @@ int path_tool_is_left(const struct path *path)
 	const struct point *prev, *p, *next;
 	double a = 0;
 
-	assert(path_is_closed(path));
+	
+	assert_path_is_closed(path);
 	prev = path->first;
 	for (p = path->first->next; p; p = p->next) {
 		next = p->next ? p->next : path->first->next;
@@ -335,7 +347,7 @@ struct path *path_offset(const struct path *path, int left, int notch)
 	struct point *n, *n2;
 	int dog;
 
-	assert(path_is_closed(path));
+	assert_path_is_closed(path);
 	if (path->first == path->last)
 		return circle(path->first->x, path->first->y, path->first->z,
 		    path->r_tool, path->r_tool, 0.1, path->id);