diff --git a/src/gc.c b/src/gc.c
index e1f0ce0b9..2110e14eb 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -371,8 +371,23 @@ mrb_gc_destroy(mrb_state *mrb, mrb_gc *gc)
 #endif
 }
 
+#ifdef MRB_GC_FIXED_ARENA
+#define gc_arena_keep(mrb, gc) /* empty */
+#else
 static void
 gc_arena_keep(mrb_state *mrb, mrb_gc *gc)
+{
+  if (gc->arena_idx >= gc->arena_capa) {
+    /* extend arena */
+    int newcapa = gc->arena_capa * 3 / 2;
+    gc->arena = (struct RBasic**)mrb_realloc(mrb, gc->arena, sizeof(struct RBasic*)*newcapa);
+    gc->arena_capa = newcapa;
+  }
+}
+#endif
+
+static inline void
+gc_protect(mrb_state *mrb, mrb_gc *gc, struct RBasic *p)
 {
 #ifdef MRB_GC_FIXED_ARENA
   if (gc->arena_idx >= MRB_GC_ARENA_SIZE) {
@@ -380,24 +395,8 @@ gc_arena_keep(mrb_state *mrb, mrb_gc *gc)
     gc->arena_idx = MRB_GC_ARENA_SIZE - 4; /* force room in arena */
     mrb_exc_raise(mrb, mrb_obj_value(mrb->arena_err));
   }
-#else
-  if (gc->arena_idx >= gc->arena_capa) {
-    /* extend arena */
-    int newcapa = gc->arena_capa * 3 / 2;
-    gc->arena = (struct RBasic**)mrb_realloc(mrb, gc->arena, sizeof(struct RBasic*)*newcapa);
-    gc->arena_capa = newcapa;
-  }
-#endif
-}
-
-static inline void
-gc_protect(mrb_state *mrb, mrb_gc *gc, struct RBasic *p)
-{
-#ifdef MRB_GC_FIXED_ARENA
-  mrb_assert(gc->arena_idx < MRB_GC_ARENA_SIZE);
-#else
-  mrb_assert(gc->arena_idx < gc->arena_capa);
 #endif
+  gc_arena_keep(mrb, gc);
   gc->arena[gc->arena_idx++] = p;
 }
 
@@ -408,7 +407,6 @@ mrb_gc_protect(mrb_state *mrb, mrb_value obj)
   if (mrb_immediate_p(obj)) return;
   struct RBasic *p = mrb_basic_ptr(obj);
   if (is_red(p)) return;
-  gc_arena_keep(mrb, &mrb->gc);
   gc_protect(mrb, &mrb->gc, p);
 }