diff --git a/method.h b/method.h
index 3aa4deb2f8..cd59963236 100644
--- a/method.h
+++ b/method.h
@@ -70,11 +70,11 @@ typedef struct rb_callable_method_entry_struct { /* same fields with rb_method_e
 #define METHOD_ENTRY_VISI(me)  (rb_method_visibility_t)(((me)->flags & (IMEMO_FL_USER0 | IMEMO_FL_USER1)) >> (IMEMO_FL_USHIFT+0))
 #define METHOD_ENTRY_BASIC(me) (int)                   (((me)->flags & (IMEMO_FL_USER2                 )) >> (IMEMO_FL_USHIFT+2))
 #define METHOD_ENTRY_COMPLEMENTED(me)        ((me)->flags & IMEMO_FL_USER3)
-#define METHOD_ENTRY_COMPLEMENTED_SET(me)    ((me)->flags = (me)->flags | IMEMO_FL_USER3)
+#define METHOD_ENTRY_COMPLEMENTED_SET(me)    ((me)->flags |= IMEMO_FL_USER3)
 #define METHOD_ENTRY_CACHED(me)              ((me)->flags & IMEMO_FL_USER4)
-#define METHOD_ENTRY_CACHED_SET(me)          ((me)->flags = (me)->flags | IMEMO_FL_USER4)
+#define METHOD_ENTRY_CACHED_SET(me)          ((me)->flags |= IMEMO_FL_USER4)
 #define METHOD_ENTRY_INVALIDATED(me)         ((me)->flags & IMEMO_FL_USER5)
-#define METHOD_ENTRY_INVALIDATED_SET(me)     ((me)->flags = (me)->flags | IMEMO_FL_USER5)
+#define METHOD_ENTRY_INVALIDATED_SET(me)     ((me)->flags |= IMEMO_FL_USER5)
 
 static inline void
 METHOD_ENTRY_VISI_SET(rb_method_entry_t *me, rb_method_visibility_t visi)