diff --git bootstraptest/test_block.rb bootstraptest/test_block.rb
index cdc5960..4347fdd 100644
--- bootstraptest/test_block.rb
+++ bootstraptest/test_block.rb
@@ -611,3 +611,17 @@ assert_equal 'ok', %q{
   end
   'ok'
 }
+
+assert_equal 'ok', %q{
+  class NilClass
+    def to_proc
+      Proc.new {|x| x }
+    end
+  end
+
+  def foo(&blk)
+    blk.call('ok')
+  end
+
+  foo(&nil)
+}, '[ruby-core:68384] [Bug #10929]'
diff --git vm_args.c vm_args.c
index cf7256b..e1cb8ac 100644
--- vm_args.c
+++ vm_args.c
@@ -755,7 +755,7 @@ vm_caller_setup_arg_block(const rb_thread_t *th, rb_control_frame_t *reg_cfp, rb
 
 	proc = *(--reg_cfp->sp);
 
-	if (proc != Qnil) {
+	if (proc != Qnil || rb_respond_to(Qnil, idTo_proc)) {
 	    if (!rb_obj_is_proc(proc)) {
 		VALUE b;