diff --git a/pinos/client/mapper.c b/pinos/client/mapper.c
index f0acba99b..511f5fb2d 100644
--- a/pinos/client/mapper.c
+++ b/pinos/client/mapper.c
@@ -62,11 +62,19 @@ id_map_get_uri (SpaIDMap *map, uint32_t id)
   return NULL;
 }
 
+static uint32_t
+id_map_get_size (SpaIDMap *map)
+{
+  IDMap *this = SPA_CONTAINER_OF (map, IDMap, map);
+  return pinos_map_get_size (&this->uris);
+}
+
 static IDMap default_id_map = {
   { sizeof (SpaIDMap),
     NULL,
     id_map_get_id,
     id_map_get_uri,
+    id_map_get_size,
   },
   { { NULL, } },
 };
diff --git a/spa/include/spa/id-map.h b/spa/include/spa/id-map.h
index 5dfb08c23..fe69bbad3 100644
--- a/spa/include/spa/id-map.h
+++ b/spa/include/spa/id-map.h
@@ -52,10 +52,13 @@ struct _SpaIDMap {
 
   const char *  (*get_uri)   (SpaIDMap   *map,
                               uint32_t    id);
+
+  uint32_t      (*get_size)  (SpaIDMap   *map);
 };
 
 #define spa_id_map_get_id(n,...)            (n)->get_id((n),__VA_ARGS__)
 #define spa_id_map_get_uri(n,...)           (n)->get_uri((n),__VA_ARGS__)
+#define spa_id_map_get_size(n)              (n)->get_size(n)
 
 #ifdef __cplusplus
 }  /* extern "C" */