Index: src/video/ZMBVEncoder.cc
--- src/video/ZMBVEncoder.cc.orig
+++ src/video/ZMBVEncoder.cc
@@ -430,7 +430,7 @@ span<const uint8_t> ZMBVEncoder::compressFrame(bool ke
 	auto r = deflate(&zstream, Z_SYNC_FLUSH);
 	assert(r == Z_OK); (void)r;
 
-	return {output.data(), writeDone + zstream.total_out};
+	return {output.data(), static_cast<size_t>(writeDone + zstream.total_out)};
 }
 
 } // namespace openmsx