diff --git a/json_object.c b/json_object.c
index 8d4d7d4..c5e72e4 100644
--- a/json_object.c
+++ b/json_object.c
@@ -377,7 +377,7 @@ static void indent(struct printbuf *pb, int level, int flags)
 
 /* json_object_object */
 
-static void json_object_object_to_json_string(struct json_object* jso,
+static int json_object_object_to_json_string(struct json_object* jso,
 					     struct printbuf *pb,
 					     int level,
 						 int flags)
@@ -421,6 +421,7 @@ static void json_object_object_to_json_string(struct json_object* jso,
 		printbuf_memappend_no_nul(pb, /*{*/ " }", 2);
 	else
 		printbuf_memappend_no_nul(pb, /*{*/ "}", 1);
+	return 0; /* we need to keep compatible with the API */
 }
 
 
@@ -553,7 +554,7 @@ void json_object_object_del(struct json_object* jso, const char *key)
 
 /* json_object_boolean */
 
-static void json_object_boolean_to_json_string(struct json_object* jso,
+static int json_object_boolean_to_json_string(struct json_object* jso,
 					      struct printbuf *pb,
 					      int level,
 						  int flags)
@@ -562,6 +563,7 @@ static void json_object_boolean_to_json_string(struct json_object* jso,
 		printbuf_memappend_no_nul(pb, "true", 4);
 	else
 		printbuf_memappend_no_nul(pb, "false", 5);
+	return 0; /* we need to keep compatible with the API */
 }
 
 struct json_object* json_object_new_boolean(json_bool b)
@@ -596,12 +598,13 @@ json_bool json_object_get_boolean(struct json_object *jso)
 
 /* json_object_int */
 
-static void json_object_int_to_json_string(struct json_object* jso,
+static int json_object_int_to_json_string(struct json_object* jso,
 					  struct printbuf *pb,
 					  int level,
 					  int flags)
 {
 	sprintbuf(pb, "%" PRId64, jso->o.c_int64);
+	return 0; /* we need to keep compatible with the API */
 }
 
 struct json_object* json_object_new_int(int32_t i)
@@ -688,7 +691,7 @@ int64_t json_object_get_int64(struct json_object *jso)
 
 /* json_object_double */
 
-static void json_object_double_to_json_string(struct json_object* jso,
+static int json_object_double_to_json_string(struct json_object* jso,
 					     struct printbuf *pb,
 					     int level,
 						 int flags)
@@ -726,6 +729,7 @@ static void json_object_double_to_json_string(struct json_object* jso,
     size = p-buf;
   }
   printbuf_memappend_no_nul(pb, buf, size);
+  return 0; /* we need to keep compatible with the API */
 }
 
 struct json_object* json_object_new_double(double d)
@@ -756,11 +760,12 @@ struct json_object* json_object_new_double_s(double d, const char *ds)
 	return jso;
 }
 
-void json_object_userdata_to_json_string(struct json_object *jso,
+int json_object_userdata_to_json_string(struct json_object *jso,
 	struct printbuf *pb, int level, int flags)
 {
 	int userdata_len = strlen((const char *)jso->_userdata);
 	printbuf_memappend_no_nul(pb, (const char *)jso->_userdata, userdata_len);
+	return 0; /* we need to keep compatible with the API */
 }
 
 void json_object_free_userdata(struct json_object *jso, void *userdata)
@@ -820,7 +825,7 @@ double json_object_get_double(struct json_object *jso)
 
 /* json_object_string */
 
-static void json_object_string_to_json_string(struct json_object* jso,
+static int json_object_string_to_json_string(struct json_object* jso,
 					     struct printbuf *pb,
 					     int level,
 						 int flags)
@@ -828,6 +833,7 @@ static void json_object_string_to_json_string(struct json_object* jso,
 	printbuf_memappend_no_nul(pb, "\"", 1);
 	json_escape_str(pb, get_string_component(jso), jso->o.c_string.len);
 	printbuf_memappend_no_nul(pb, "\"", 1);
+	return 0; /* we need to keep compatible with the API */
 }
 
 static void json_object_string_delete(struct json_object* jso)
@@ -914,7 +920,7 @@ int json_object_get_string_len(struct json_object *jso)
 
 /* json_object_array */
 
-static void json_object_array_to_json_string(struct json_object* jso,
+static int json_object_array_to_json_string(struct json_object* jso,
                                             struct printbuf *pb,
                                             int level,
                                             int flags)
@@ -954,6 +960,7 @@ static void json_object_array_to_json_string(struct json_object* jso,
 		printbuf_memappend_no_nul(pb, " ]", 2);
 	else
 		printbuf_memappend_no_nul(pb, "]", 1);
+	return 0; /* we need to keep compatible with the API */
 }
 
 static void json_object_array_entry_free(void *data)
diff --git a/json_object.h b/json_object.h
index ee6b0b2..67137d8 100644
--- a/json_object.h
+++ b/json_object.h
@@ -128,7 +128,7 @@ typedef void (json_object_delete_fn)(struct json_object *jso, void *userdata);
 /**
  * Type of a custom serialization function.  See json_object_set_serializer.
  */
-typedef void (json_object_to_json_string_fn)(struct json_object *jso,
+typedef int (json_object_to_json_string_fn)(struct json_object *jso,
 						struct printbuf *pb,
 						int level,
 						int flags);