diff --git a/src/liblzma/lz/lz_encoder.h b/src/liblzma/lz/lz_encoder.h
index 4904c229..35e9ebbf 100644
--- a/src/liblzma/lz/lz_encoder.h
+++ b/src/liblzma/lz/lz_encoder.h
@@ -234,7 +234,7 @@ mf_avail(const lzma_mf *mf)
 static inline uint32_t
 mf_unencoded(const lzma_mf *mf)
 {
-	return mf->write_pos - mf->read_pos - mf->read_ahead;
+	return mf->write_pos - mf->read_pos + mf->read_ahead;
 }