diff --git a/ChangeLog b/ChangeLog
index 176b97bdb0..fa969d6230 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jul 16 11:12:03 2013  Masaki Matsushita  <glass.saga@gmail.com>
+
+	* proc.c (rb_block_arity): raise ArgumentError if no block given.
+
 Tue Jul 16 08:15:22 2013  Zachary Scott  <e@zzak.io>
 
 	* ext/bigdecimal/lib/bigdecimal/util.rb: [DOC] document top-level
diff --git a/proc.c b/proc.c
index 63bed7257e..bcdcb3607c 100644
--- a/proc.c
+++ b/proc.c
@@ -722,9 +722,11 @@ rb_block_arity(void)
     rb_thread_t *th = GET_THREAD();
     rb_control_frame_t *cfp = th->cfp;
     rb_block_t *block = rb_vm_control_frame_block_ptr(cfp);
-    VALUE proc_value = block->proc;
+    VALUE proc_value;
 
+    if (!block) rb_raise(rb_eArgError, "no block given");
     min = rb_block_min_max_arity(block, &max);
+    proc_value = block->proc;
     if (proc_value) {
 	rb_proc_t *proc;
 	GetProcPtr(proc_value, proc);