diff --git a/protobuf/src/coded_output_stream/mod.rs b/protobuf/src/coded_output_stream/mod.rs
index 26293399f..71ce1568a 100644
--- a/protobuf/src/coded_output_stream/mod.rs
+++ b/protobuf/src/coded_output_stream/mod.rs
@@ -244,6 +244,7 @@ impl<'a> CodedOutputStream<'a> {
             }
             OutputTarget::Write(ref mut write, _) => {
                 write.write_all(bytes)?;
+                self.pos_of_buffer_start += bytes.len() as u64;
             }
             OutputTarget::Vec(ref mut vec) => {
                 assert!(self.buffer.pos_within_buf() == 0);