diff --git a/src/threadpool.h b/src/threadpool.h
index eb028b9162a9a231291d06854de0c7be0db8b71a..02eacf9317cf197b610fef09f8337fab4a106cd6 100644
--- a/src/threadpool.h
+++ b/src/threadpool.h
@@ -42,7 +42,7 @@ struct threadpool {
   pthread_cond_t control_cond, thread_cond;
 
   /* Current map data and count. */
-  void *map_data;
+  void *map_data, *map_extra_data;
   size_t map_data_count, map_data_size, map_data_stride;
   threadpool_map_function map_function;