diff --git a/gc.c b/gc.c
index bd3915fb47..b5d5df692b 100644
--- a/gc.c
+++ b/gc.c
@@ -2331,7 +2331,7 @@ rb_gc_obj_slot_size(VALUE obj)
 }
 
 static inline size_t
-size_pool_slot_size(char pool_id)
+size_pool_slot_size(unsigned char pool_id)
 {
     GC_ASSERT(pool_id < SIZE_POOL_COUNT);