diff --git a/contrib/20.r7rs/src/system.c b/contrib/20.r7rs/src/system.c
index 12e512dc..63f6f0a4 100644
--- a/contrib/20.r7rs/src/system.c
+++ b/contrib/20.r7rs/src/system.c
@@ -17,7 +17,7 @@ pic_system_cmdline(pic_state *pic)
   for (i = 0; i < pic->argc; ++i) {
     size_t ai = pic_gc_arena_preserve(pic);
 
-    v = pic_cons(pic, pic_obj_value(pic_make_str_cstr(pic, pic->argv[i])), v);
+    v = pic_cons(pic, pic_obj_value(pic_make_cstr(pic, pic->argv[i])), v);
     pic_gc_arena_restore(pic, ai);
   }
 
@@ -84,7 +84,7 @@ pic_system_getenv(pic_state *pic)
   if (val == NULL)
     return pic_nil_value();
   else
-    return pic_obj_value(pic_make_str_cstr(pic, val));
+    return pic_obj_value(pic_make_cstr(pic, val));
 }
 
 static pic_value
@@ -108,7 +108,7 @@ pic_system_getenvs(pic_state *pic)
       ;
 
     key = pic_make_str(pic, *envp, i);
-    val = pic_make_str_cstr(pic, getenv(pic_str_cstr(pic, key)));
+    val = pic_make_cstr(pic, getenv(pic_str_cstr(pic, key)));
 
     /* push */
     data = pic_acons(pic, pic_obj_value(key), pic_obj_value(val), data);
diff --git a/contrib/30.readline/src/readline.c b/contrib/30.readline/src/readline.c
index 84d3f37f..d6e71d6a 100644
--- a/contrib/30.readline/src/readline.c
+++ b/contrib/30.readline/src/readline.c
@@ -19,7 +19,7 @@ pic_rl_readline(pic_state *pic)
   result = readline(prompt);
 
   if(result)
-    return pic_obj_value(pic_make_str_cstr(pic, result));
+    return pic_obj_value(pic_make_cstr(pic, result));
   else
     return pic_eof_object();
 }
@@ -87,7 +87,7 @@ pic_rl_current_history(pic_state *pic)
 {
   pic_get_args(pic, "");
 
-  return pic_obj_value(pic_make_str_cstr(pic, current_history()->line));
+  return pic_obj_value(pic_make_cstr(pic, current_history()->line));
 }
 
 static pic_value
@@ -100,7 +100,7 @@ pic_rl_history_get(pic_state *pic)
   
   e = history_get(i);
 
-  return e ? pic_obj_value(pic_make_str_cstr(pic, e->line))
+  return e ? pic_obj_value(pic_make_cstr(pic, e->line))
     : pic_false_value();
 }
 
@@ -114,7 +114,7 @@ pic_rl_remove_history(pic_state *pic)
   
   e = remove_history(i);
 
-  return e ? pic_obj_value(pic_make_str_cstr(pic, e->line))
+  return e ? pic_obj_value(pic_make_cstr(pic, e->line))
     : pic_false_value();
 }
 
@@ -148,7 +148,7 @@ pic_rl_previous_history(pic_state *pic)
 
   e = previous_history();
 
-  return e ? pic_obj_value(pic_make_str_cstr(pic, e->line))
+  return e ? pic_obj_value(pic_make_cstr(pic, e->line))
     : pic_false_value();
 }
 
@@ -161,7 +161,7 @@ pic_rl_next_history(pic_state *pic)
 
   e = next_history();
 
-  return e ? pic_obj_value(pic_make_str_cstr(pic, e->line))
+  return e ? pic_obj_value(pic_make_cstr(pic, e->line))
     : pic_false_value();
 }
 
@@ -240,7 +240,7 @@ pic_rl_history_expand(pic_state *pic)
   if(status == -1 || status == 2)
     pic_errorf(pic, "%s\n", result);
 
-  return pic_obj_value(pic_make_str_cstr(pic, result));
+  return pic_obj_value(pic_make_cstr(pic, result));
 }
 
 void
diff --git a/contrib/30.regexp/src/regexp.c b/contrib/30.regexp/src/regexp.c
index ce54d65e..2af663dd 100644
--- a/contrib/30.regexp/src/regexp.c
+++ b/contrib/30.regexp/src/regexp.c
@@ -146,7 +146,7 @@ pic_regexp_regexp_split(pic_state *pic)
     input += match.rm_eo;
   }
 
-  pic_push(pic, pic_obj_value(pic_make_str_cstr(pic, input)), output);
+  pic_push(pic, pic_obj_value(pic_make_cstr(pic, input)), output);
 
   return pic_reverse(pic, output);
 }
@@ -157,7 +157,7 @@ pic_regexp_regexp_replace(pic_state *pic)
   pic_value reg;
   const char *input;
   regmatch_t match;
-  pic_str *txt, *output = pic_make_str(pic, NULL, 0);
+  pic_str *txt, *output = pic_make_lit(pic, "");
 
   pic_get_args(pic, "ozs", &reg, &input, &txt);
 
diff --git a/extlib/benz/debug.c b/extlib/benz/debug.c
index 7d682f0f..9799692c 100644
--- a/extlib/benz/debug.c
+++ b/extlib/benz/debug.c
@@ -11,18 +11,18 @@ pic_get_backtrace(pic_state *pic)
   pic_callinfo *ci;
   pic_str *trace;
 
-  trace = pic_make_str(pic, NULL, 0);
+  trace = pic_make_lit(pic, "");
 
   for (ci = pic->ci; ci != pic->cibase; --ci) {
     struct pic_proc *proc = pic_proc_ptr(ci->fp[0]);
 
-    trace = pic_str_cat(pic, trace, pic_make_str_cstr(pic, "  at "));
-    trace = pic_str_cat(pic, trace, pic_make_str_cstr(pic, "(anonymous lambda)"));
+    trace = pic_str_cat(pic, trace, pic_make_lit(pic, "  at "));
+    trace = pic_str_cat(pic, trace, pic_make_lit(pic, "(anonymous lambda)"));
 
     if (pic_proc_func_p(proc)) {
-      trace = pic_str_cat(pic, trace, pic_make_str_cstr(pic, " (native function)\n"));
+      trace = pic_str_cat(pic, trace, pic_make_lit(pic, " (native function)\n"));
     } else if (pic_proc_irep_p(proc)) {
-      trace = pic_str_cat(pic, trace, pic_make_str_cstr(pic, " (unknown location)\n")); /* TODO */
+      trace = pic_str_cat(pic, trace, pic_make_lit(pic, " (unknown location)\n")); /* TODO */
     }
   }
 
diff --git a/extlib/benz/error.c b/extlib/benz/error.c
index 42d3251c..1010b7fe 100644
--- a/extlib/benz/error.c
+++ b/extlib/benz/error.c
@@ -100,7 +100,7 @@ pic_make_error(pic_state *pic, pic_sym *type, const char *msg, pic_value irrs)
 
   e = (struct pic_error *)pic_obj_alloc(pic, sizeof(struct pic_error), PIC_TT_ERROR);
   e->type = type;
-  e->msg = pic_make_str_cstr(pic, msg);
+  e->msg = pic_make_cstr(pic, msg);
   e->irrs = irrs;
   e->stack = stack;
 
diff --git a/extlib/benz/include/picrin/string.h b/extlib/benz/include/picrin/string.h
index 36389412..2f5e8361 100644
--- a/extlib/benz/include/picrin/string.h
+++ b/extlib/benz/include/picrin/string.h
@@ -20,8 +20,9 @@ void pic_rope_decref(pic_state *, struct pic_rope *);
 #define pic_str_p(v) (pic_type(v) == PIC_TT_STRING)
 #define pic_str_ptr(o) ((struct pic_string *)pic_ptr(o))
 
-pic_str *pic_make_str(pic_state *, const char * /* nullable */, int);
-pic_str *pic_make_str_cstr(pic_state *, const char *);
+pic_str *pic_make_str(pic_state *, const char *, int);
+#define pic_make_cstr(pic, cstr) pic_make_str(pic, (cstr), strlen(cstr))
+#define pic_make_lit(pic, lit) pic_make_str(pic, "" lit, -((int)sizeof lit - 1))
 
 char pic_str_ref(pic_state *, pic_str *, int);
 int pic_str_len(pic_str *);
diff --git a/extlib/benz/string.c b/extlib/benz/string.c
index a5ab3191..0df0d631 100644
--- a/extlib/benz/string.c
+++ b/extlib/benz/string.c
@@ -26,9 +26,6 @@ struct pic_rope {
 #define CHUNK_DECREF(c) do {                    \
     struct pic_chunk *c_ = (c);                 \
     if (! --c_->refcnt) {                       \
-      if (c_->str != c_->buf) {                 \
-        pic_free(pic, c_->str);                 \
-      }                                         \
       pic_free(pic, c_);                        \
     }                                           \
   } while (0)
@@ -67,6 +64,19 @@ pic_make_chunk(pic_state *pic, const char *str, size_t len)
   return c;
 }
 
+static struct pic_chunk *
+pic_make_chunk_lit(pic_state *pic, const char *str, size_t len)
+{
+  struct pic_chunk *c;
+
+  c = pic_malloc(pic, sizeof(struct pic_chunk));
+  c->refcnt = 1;
+  c->str = (char *)str;
+  c->len = len;
+
+  return c;
+}
+
 static struct pic_rope *
 pic_make_rope(pic_state *pic, struct pic_chunk *c)
 {
@@ -214,7 +224,7 @@ rope_cstr(pic_state *pic, struct pic_rope *x)
     return x->chunk->str;       /* reuse cached chunk */
   }
 
-  c = pic_malloc(pic, sizeof(struct pic_chunk) + x->weight);
+  c = pic_malloc(pic, offsetof(struct pic_chunk, buf) + x->weight + 1);
   c->refcnt = 1;
   c->len = x->weight;
   c->str = c->buf;
@@ -229,16 +239,17 @@ rope_cstr(pic_state *pic, struct pic_rope *x)
 pic_str *
 pic_make_str(pic_state *pic, const char *str, int len)
 {
-  if (str == NULL && len > 0) {
-    pic_errorf(pic, "zero length specified against NULL ptr");
-  }
-  return pic_make_string(pic, pic_make_rope(pic, pic_make_chunk(pic, str, len)));
-}
+  struct pic_chunk *c;
 
-pic_str *
-pic_make_str_cstr(pic_state *pic, const char *cstr)
-{
-  return pic_make_str(pic, cstr, strlen(cstr));
+  if (len > 0) {
+    c = pic_make_chunk(pic, str, len);
+  } else {
+    if (len == 0) {
+      str = "";
+    }
+    c = pic_make_chunk_lit(pic, str, -len);
+  }
+  return pic_make_string(pic, pic_make_rope(pic, c));
 }
 
 int
@@ -512,7 +523,7 @@ pic_str_string_append(pic_state *pic)
 
   pic_get_args(pic, "*", &argc, &argv);
 
-  str = pic_make_str(pic, NULL, 0);
+  str = pic_make_lit(pic, "");
   for (i = 0; i < argc; ++i) {
     if (! pic_str_p(argv[i])) {
       pic_errorf(pic, "type error");
@@ -616,7 +627,7 @@ pic_str_list_to_string(pic_state *pic)
   pic_get_args(pic, "o", &list);
 
   if (pic_length(pic, list) == 0) {
-    return pic_obj_value(pic_make_str(pic, NULL, 0));
+    return pic_obj_value(pic_make_lit(pic, ""));
   }
 
   buf = pic_malloc(pic, pic_length(pic, list));
diff --git a/extlib/benz/symbol.c b/extlib/benz/symbol.c
index f42c31cf..3edc4f4a 100644
--- a/extlib/benz/symbol.c
+++ b/extlib/benz/symbol.c
@@ -104,7 +104,7 @@ pic_symbol_symbol_to_string(pic_state *pic)
 
   pic_get_args(pic, "m", &sym);
 
-  return pic_obj_value(pic_make_str_cstr(pic, sym->cstr));
+  return pic_obj_value(pic_make_cstr(pic, sym->cstr));
 }
 
 static pic_value