diff --git a/src/lib/curl/Headers.hxx b/src/lib/curl/Headers.hxx
index 7ff04a6e4..2d7168411 100644
--- a/src/lib/curl/Headers.hxx
+++ b/src/lib/curl/Headers.hxx
@@ -37,6 +37,6 @@
 
 namespace Curl {
 
-using Headers = std::multimap<std::string, std::string>;
+using Headers = std::multimap<std::string, std::string, std::less<>>;
 
 } // namespace Curl