diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1c3e8a95..5ec5e13e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 
+- Fix `-Wsign-conversion` warning in `basic_pbf_writer::commit_submessage()`.
+
 ## [1.8.2] - 2026-06-30
 
 ### Fixed
diff --git a/include/protozero/basic_pbf_writer.hpp b/include/protozero/basic_pbf_writer.hpp
index cc0f7cd5..9e0af4ea 100644
--- a/include/protozero/basic_pbf_writer.hpp
+++ b/include/protozero/basic_pbf_writer.hpp
@@ -204,7 +204,7 @@ class basic_pbf_writer {
         protozero_assert(buffer_customization<TBuffer>::size(m_data) >= m_pos - reserve_bytes);
         const auto n = add_varint_to_buffer(buffer_customization<TBuffer>::at_pos(m_data, m_pos - reserve_bytes), length);
 
-        buffer_customization<TBuffer>::erase_range(m_data, m_pos - reserve_bytes + n, m_pos);
+        buffer_customization<TBuffer>::erase_range(m_data, m_pos - reserve_bytes + static_cast<std::size_t>(n), m_pos);
         m_pos = 0;
     }