diff --git a/include/r3_str.h b/include/r3_str.h
index e07eb21..1504486 100644
--- a/include/r3_str.h
+++ b/include/r3_str.h
@@ -10,7 +10,7 @@
 #include "r3.h"
 #include "config.h"
 
-int slug_count(const char * p, int len);
+int slug_count(const char * p, int len, char ** errstr);
 
 char * slug_compile(const char * str, int len);
 
@@ -20,7 +20,7 @@ char * slug_find_pattern(const char *s1, int *len);
 
 char * slug_find_placeholder(const char *s1, int *len);
 
-char * inside_slug(const char * needle, int needle_len, char *offset);
+char * inside_slug(const char * needle, int needle_len, char *offset, char **errstr);
 
 char * ltrim_slash(char* str);
 
diff --git a/src/node.c b/src/node.c
index 2500aee..08cba95 100644
--- a/src/node.c
+++ b/src/node.c
@@ -493,7 +493,7 @@ node * r3_tree_insert_pathl_(node *tree, const char *path, int path_len, route *
 
     // branch the edge at correct position (avoid broken slugs)
     const char *slug_s;
-    if ( (slug_s = inside_slug(path, path_len, (char*) path + prefix_len)) != NULL ) {
+    if ( (slug_s = inside_slug(path, path_len, ((char*) path + prefix_len), NULL)) != NULL ) {
         prefix_len = slug_s - path;
     }
 
@@ -503,7 +503,9 @@ node * r3_tree_insert_pathl_(node *tree, const char *path, int path_len, route *
     // common prefix not found, insert a new edge for this pattern
     if ( prefix_len == 0 ) {
         // there are two more slugs, we should break them into several parts
-        int slug_cnt = slug_count(path, path_len);
+        char *errstr = NULL;
+        int slug_cnt = slug_count(path, path_len, &errstr);
+
         if ( slug_cnt > 1 ) {
             int   slug_len;
             char *p = slug_find_placeholder(path, &slug_len);
diff --git a/src/str.c b/src/str.c
index cdfdf57..c2e7721 100644
--- a/src/str.c
+++ b/src/str.c
@@ -42,7 +42,7 @@ int r3_pattern_to_opcode(const char * pattern, int len) {
 /**
  * provide a quick way to count slugs, simply search for '{'
  */
-int slug_count(const char * p, int len) {
+int slug_count(const char * p, int len, char **errstr) {
     int s = 0;
     int lev = 0;
     while( len-- ) {
@@ -56,6 +56,15 @@ int slug_count(const char * p, int len) {
         }
         p++;
     }
+    /*
+    XXX:
+    if (lev != 0) {
+        if (errstr) {
+            asprintf(errstr, "incomplete slug pattern: %s", p);
+        }
+        return 0;
+    }
+    */
     return s;
 }
 
@@ -63,7 +72,7 @@ bool contains_slug(const char * str) {
     return strchr(str, '{') != NULL ? TRUE : FALSE;
 }
 
-char * inside_slug(const char * needle, int needle_len, char *offset) {
+char * inside_slug(const char * needle, int needle_len, char *offset, char **errstr) {
     char * s1 = offset;
     char * s2 = offset;
 
@@ -89,6 +98,13 @@ char * inside_slug(const char * needle, int needle_len, char *offset) {
     if (found_s1 && found_s2) {
         return s1;
     }
+    if (found_s1 || found_s2) {
+        // wrong slug pattern
+        if(errstr) {
+            asprintf(errstr, "incomplete slug pattern");
+        }
+        return NULL;
+    }
     return NULL;
 }
 
diff --git a/tests/check_slug.c b/tests/check_slug.c
index 5fc362a..ca42f85 100644
--- a/tests/check_slug.c
+++ b/tests/check_slug.c
@@ -76,19 +76,19 @@ START_TEST (test_inside_slug)
 {
     char * pattern = "/user/{name:\\s+}/to/{id}";
     char * offset = strchr(pattern, '{') + 2;
-    ck_assert( (int)inside_slug(pattern, strlen(pattern), offset) );
-    ck_assert( *(inside_slug(pattern, strlen(pattern), offset)) == '{' );
-    ck_assert( ! inside_slug(pattern, strlen(pattern), pattern) );
+    ck_assert( (int)inside_slug(pattern, strlen(pattern), offset, NULL) );
+    ck_assert( *(inside_slug(pattern, strlen(pattern), offset, NULL)) == '{' );
+    ck_assert( ! inside_slug(pattern, strlen(pattern), pattern, NULL) );
 }
 END_TEST
 
 START_TEST (test_slug_count)
 {
     char * pattern = "/user/{name:\\s+}/to/{id}";
-    ck_assert_int_eq( slug_count(pattern, strlen(pattern) ), 2 );
+    ck_assert_int_eq( slug_count(pattern, strlen(pattern), NULL), 2 );
 
     char * pattern2 = "/user/{name:\\d{3}}/to/{id}";
-    ck_assert_int_eq( slug_count(pattern2, strlen(pattern) ), 2 );
+    ck_assert_int_eq( slug_count(pattern2, strlen(pattern), NULL), 2 );
 }
 END_TEST