diff --git a/include/picrin/macro.h b/include/picrin/macro.h
index 75257708..5dcf2c5d 100644
--- a/include/picrin/macro.h
+++ b/include/picrin/macro.h
@@ -6,6 +6,7 @@ struct pic_senv {
   struct pic_senv *up;
   struct xhash *tbl;
   struct pic_syntax **stx;
+  size_t xlen, xcapa;
 };
 
 struct pic_syntax {
diff --git a/src/gc.c b/src/gc.c
index fc2f38f2..2264d5bc 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -287,7 +287,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     if (senv->stx) {
       int i;
 
-      for (i = 0; i < 6; ++i) {
+      for (i = 0; i < senv->xlen; ++i) {
 	gc_mark_object(pic, (struct pic_object *)senv->stx[i]);
       }
     }
diff --git a/src/macro.c b/src/macro.c
index 84552119..cac35c3d 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -292,6 +292,8 @@ macroexpand(pic_state *pic, pic_value expr, struct pic_senv *senv)
 	  in->up = senv;
 	  in->tbl = xh_new();
 	  in->stx = NULL;
+	  in->xlen = 0;
+	  in->xcapa = 0;
 
 	  for (a = var; pic_pair_p(a); a = pic_cdr(pic, a)) {
 	    pic_sym gen, orig;
@@ -395,6 +397,8 @@ pic_macroexpand_2(pic_state *pic, pic_value expr)
   senv->up = NULL;
   senv->tbl = pic->var_tbl;
   senv->stx = pic->stx;
+  senv->xlen = pic->xlen;
+  senv->xcapa = pic->xcapa;
 
   return macroexpand(pic, expr, senv);
 }