diff --git a/lib/rbtrace/rbtracer.rb b/lib/rbtrace/rbtracer.rb
index a672d9b..b85a663 100644
--- a/lib/rbtrace/rbtracer.rb
+++ b/lib/rbtrace/rbtracer.rb
@@ -323,7 +323,7 @@ def send_cmd(*cmd)
       msg = cmd.to_msgpack
       # A message is null-terminated, but bytesize gives the unterminated
       # length.
-      if msg.bytesize >= RbTrace::BUF_SIZE
+      if msg.bytesize >= RBTrace::BUF_SIZE
         raise ArgumentError, "command is too long (#{msg.bytesize}B >= #{MsgQ::EventMsg::BUF_SIZE}B)"
       end
       MsgQ::EventMsg.send_cmd(@qo, msg)