diff --git a/src/anbox/qemu/qemud_message_processor.cpp b/src/anbox/qemu/qemud_message_processor.cpp
index d9ee009..b47f078 100644
--- a/src/anbox/qemu/qemud_message_processor.cpp
+++ b/src/anbox/qemu/qemud_message_processor.cpp
@@ -70,7 +70,7 @@ void QemudMessageProcessor::process_commands() {
 
 void QemudMessageProcessor::send_header(const size_t &size) {
   char header[header_size + 1];
-  std::snprintf(header, header_size + 1, "%04lx", size);
+  std::snprintf(header, header_size + 1, "%04zx", size);
   messenger_->send(header, header_size);
 }