diff --git a/lz4diff/lz4patch.h b/lz4diff/lz4patch.h
index 8f66180..ce49430 100644
--- a/lz4diff/lz4patch.h
+++ b/lz4diff/lz4patch.h
@@ -26,6 +26,8 @@
               Blob* output);
 bool Lz4Patch(const Blob& src_data, const Blob& patch_data, Blob* output);
 
+std::ostream& operator<<(std::ostream& out, const CompressionAlgorithm& info);
+
 std::ostream& operator<<(std::ostream& out, const Lz4diffHeader&);
 
 template <typename T>