diff --git a/include/r3.h b/include/r3.h
index a8568d4..6db0143 100644
--- a/include/r3.h
+++ b/include/r3.h
@@ -109,7 +109,7 @@ edge * r3_node_find_edge(const node * n, const char * pat, int pat_len);
 void r3_node_append_edge(node *n, edge *child);
 
 
-edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix_len);
+edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix_len, char **errstr);
 
 node * r3_tree_insert_pathl(node *tree, const char *path, int path_len, void * data);
 
diff --git a/src/node.c b/src/node.c
index 3cdfb9d..b02eefb 100644
--- a/src/node.c
+++ b/src/node.c
@@ -492,7 +492,7 @@ node * r3_tree_insert_pathl(node *tree, const char *path, int path_len, void * d
  * 4.  "aaa{slug:xxx}/hate" vs "aab{slug:yyy}/bar"      => common prefix = "aa"
  * 5.  "/foo/{slug}/hate" vs "/fo{slug}/bar"            => common prefix = "/fo"
  */
-edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix_len) {
+edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix_len, char **errstr) {
     int i = 0;
     int prefix = 0;
     edge *e = NULL;
@@ -520,7 +520,7 @@ edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix
         slug = r3_slug_new(path, path_len);
 
         do {
-            ret = r3_slug_parse(slug, path, path_len, offset, NULL);
+            ret = r3_slug_parse(slug, path, path_len, offset, errstr);
             // found slug
             if (ret == 1) {
                 // inside slug, backtrace to the begin of the slug
@@ -536,6 +536,8 @@ edge * r3_node_find_common_prefix(node *n, char *path, int path_len, int *prefix
                 } else {
                     break;
                 }
+            } else if (ret == -1) {
+                return -1;
             } else {
                 break;
             }
@@ -563,7 +565,10 @@ node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route
 
     /* length of common prefix */
     int prefix_len = 0;
-    e = r3_node_find_common_prefix(tree, path, path_len, &prefix_len);
+    e = r3_node_find_common_prefix(tree, path, path_len, &prefix_len, errstr);
+    if (e == -1) {
+        return NULL;
+    }
 
     const char * subpath = path + prefix_len;
     const int    subpath_len = path_len - prefix_len;
diff --git a/tests/check_slug.c b/tests/check_slug.c
index 9b05a2e..2e89136 100644
--- a/tests/check_slug.c
+++ b/tests/check_slug.c
@@ -88,7 +88,7 @@ START_TEST (test_incomplete_slug)
     char * errstr = NULL;
     char * pattern = "/user/{name:\\d{3}}/to/{id";
     cnt = slug_count(pattern, strlen(pattern), &errstr);
-    ck_assert_int_eq(cnt, 0);
+    ck_assert_int_eq(cnt, -1);
     ck_assert(errstr);
     printf("%s\n",errstr);
     free(errstr);
diff --git a/tests/check_tree.c b/tests/check_tree.c
index beae505..d376d50 100644
--- a/tests/check_tree.c
+++ b/tests/check_tree.c
@@ -20,43 +20,43 @@ START_TEST (test_find_common_prefix)
     edge *ret_edge = NULL;
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo", sizeof("/foo")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/foo", sizeof("/foo")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 4);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/", sizeof("/foo/")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/", sizeof("/foo/")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 5);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/{slog}", sizeof("/foo/{slog}")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/{slog}", sizeof("/foo/{slog}")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 5);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/{bar}", sizeof("/foo/{bar}")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/{bar}", sizeof("/foo/{bar}")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 5);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/bar", sizeof("/foo/bar")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/bar", sizeof("/foo/bar")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 5);
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/bar/", sizeof("/bar/")-1, &prefix_len);
-    ck_assert(ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "/bar/", sizeof("/bar/")-1, &prefix_len, NULL);
+    ck_assert(ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 1);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "{bar}", sizeof("{bar}")-1, &prefix_len);
-    ck_assert(!ret_edge);
+    ret_edge = r3_node_find_common_prefix(n, "{bar}", sizeof("{bar}")-1, &prefix_len, NULL);
+    ck_assert(!ret_edge != NULL);
     ck_assert_int_eq(prefix_len, 0);
 
 
@@ -79,19 +79,19 @@ START_TEST (test_find_common_prefix_after)
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo", sizeof("/foo")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "/foo", sizeof("/foo")-1, &prefix_len, NULL);
     ck_assert(ret_edge == NULL);
     ck_assert_int_eq(prefix_len, 0);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "{slug}/bar", sizeof("{slug}/bar")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "{slug}/bar", sizeof("{slug}/bar")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 7);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "{slug}/foo", sizeof("{slug}/foo")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "{slug}/foo", sizeof("{slug}/foo")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 10);
 
@@ -112,7 +112,7 @@ START_TEST (test_find_common_prefix_double_middle)
     edge *ret_edge = NULL;
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "{slug}/foo/{number}", sizeof("{slug}/foo/{number}")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "{slug}/foo/{number}", sizeof("{slug}/foo/{number}")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 11);
 
@@ -132,12 +132,12 @@ START_TEST (test_find_common_prefix_middle)
     edge *ret_edge = NULL;
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug}/bar", sizeof("/foo/{slug}/bar")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug}/bar", sizeof("/foo/{slug}/bar")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 12);
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/fo/{slug}/bar", sizeof("/fo/{slug}/bar")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "/fo/{slug}/bar", sizeof("/fo/{slug}/bar")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 3);
 
@@ -155,13 +155,13 @@ START_TEST (test_find_common_prefix_same_pattern)
     edge *ret_edge = NULL;
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug:yyy}/hate", sizeof("/foo/{slug:yyy}/hate")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug:yyy}/hate", sizeof("/foo/{slug:yyy}/hate")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 5);
 
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug}/hate", sizeof("/foo/{slug}/hate")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "/foo/{slug}/hate", sizeof("/foo/{slug}/hate")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 5);
 
@@ -179,7 +179,7 @@ START_TEST (test_find_common_prefix_same_pattern2)
     edge *ret_edge = NULL;
 
     prefix_len = 0;
-    ret_edge = r3_node_find_common_prefix(n, "{slug:yyy}/hate", sizeof("{slug:yyy}/hate")-1, &prefix_len);
+    ret_edge = r3_node_find_common_prefix(n, "{slug:yyy}/hate", sizeof("{slug:yyy}/hate")-1, &prefix_len, NULL);
     ck_assert(ret_edge);
     ck_assert_int_eq(prefix_len, 0);