diff --git a/src/v2_decoder.cpp b/src/v2_decoder.cpp
index 5781d804..9e0256db 100644
--- a/src/v2_decoder.cpp
+++ b/src/v2_decoder.cpp
@@ -115,8 +115,8 @@ int zmq::v2_decoder_t::size_ready (uint64_t msg_size_,
 
     shared_message_memory_allocator &allocator = get_allocator ();
     if (unlikely (!_zero_copy
-                  || msg_size_ >
-                  (size_t)(allocator.data () + allocator.size () - read_pos_))) {
+                  || msg_size_ > (size_t) (allocator.data () + allocator.size ()
+                                           - read_pos_))) {
         // a new message has started, but the size would exceed the pre-allocated arena
         // this happens every time when a message does not fit completely into the buffer
         rc = _in_progress.init_size (static_cast<size_t> (msg_size_));