diff --git a/libgo/sync/co_condition_variable.h b/libgo/sync/co_condition_variable.h
index 0469be68..aea15563 100644
--- a/libgo/sync/co_condition_variable.h
+++ b/libgo/sync/co_condition_variable.h
@@ -218,7 +218,7 @@ class ConditionVariableAnyT
         return queue_.empty();
     }
 
-    bool size() {
+    size_t size() {
         return queue_.size();
     }