diff --git a/include/picrin.h b/include/picrin.h
index ec593460..626e3503 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -62,7 +62,7 @@ void pic_gc_arena_restore(pic_state *, int);
 pic_state *pic_open(int argc, char *argv[], char **envp);
 void pic_close(pic_state *);
 
-void pic_get_args(pic_state *, const char *, ...);
+int pic_get_args(pic_state *, const char *, ...);
 void pic_defun(pic_state *, const char *, pic_func_t);
 
 bool pic_eq_p(pic_state *, pic_value, pic_value);
diff --git a/src/port.c b/src/port.c
index 894b2f2a..d6de83d6 100644
--- a/src/port.c
+++ b/src/port.c
@@ -110,4 +110,3 @@ pic_init_port(pic_state *pic)
   pic_defun(pic, "write", pic_port_write);
   pic_defun(pic, "newline", pic_port_newline);
 }
-
diff --git a/src/vm.c b/src/vm.c
index 2e8c5427..46b9f3bc 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -7,22 +7,38 @@
 #include "picrin/irep.h"
 #include "picrin/string.h"
 
-void
+#define GET_OPERAND(pic,n) ((pic)->sp[-1-(n)])
+
+int
 pic_get_args(pic_state *pic, const char *format, ...)
 {
   char c;
-  int i = -1;
+  int i = 0, argc = pic->ci->argc - 1;
   va_list ap;
+  bool opt = false;
 
   va_start(ap, format);
   while ((c = *format++)) {
     switch (c) {
+    default:
+      if (argc <= i && ! opt) {
+	pic_error(pic, "wrong number of arguments");
+      }
+      break;
+    case '|':
+      break;
+    }
+    switch (c) {
+    case '|':
+      opt = true;
+      break;
     case 'o':
       {
 	pic_value *p;
 
 	p = va_arg(ap, pic_value*);
-	*p = pic->sp[i--];
+	*p = GET_OPERAND(pic,i);
+	i++;
       }
       break;
     case 'f':
@@ -30,7 +46,8 @@ pic_get_args(pic_state *pic, const char *format, ...)
 	double *f;
 
 	f = va_arg(ap, double *);
-	*f = pic_float(pic->sp[i--]);
+	*f = pic_float(GET_OPERAND(pic,i));
+	i++;
       }
       break;
     case 's':
@@ -41,13 +58,19 @@ pic_get_args(pic_state *pic, const char *format, ...)
 
 	cstr = va_arg(ap, char **);
 	len = va_arg(ap, size_t *);
-	str = pic->sp[i--];
+	str = GET_OPERAND(pic,i);
 	*cstr = pic_str_ptr(str)->str;
 	*len = pic_str_ptr(str)->len;
+	i++;
       }
       break;
     }
   }
+  if (argc > i) {
+    pic_error(pic, "wrong number of arguments");
+  }
+  va_end(ap);
+  return i;
 }
 
 #if PIC_DIRECT_THREADED_VM