diff --git a/src/about.cpp b/src/about.cpp
index a4c415e7fe9..69726f1ffc7 100644
--- a/src/about.cpp
+++ b/src/about.cpp
@@ -84,7 +84,7 @@ credits_group::about_group::about_group(const config& cfg)
 	}
 }
 
-bool credits_group::about_group::operator<(const about_group& o)
+bool credits_group::about_group::operator<(const about_group& o) const
 {
 	return translation::compare(title.str(), o.title.str()) < 0;
 }
diff --git a/src/about.hpp b/src/about.hpp
index 7d92e076f0a..25c9e49a334 100644
--- a/src/about.hpp
+++ b/src/about.hpp
@@ -35,7 +35,7 @@ struct credits_group
 		/** The section title. */
 		t_string title;
 
-		bool operator<(const about_group& o);
+		bool operator<(const about_group& o) const;
 	};
 
 	credits_group(const config& cfg, bool is_campaign_credits);