diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 5c05d8fa85..a9eedcef16 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -548,7 +548,7 @@ void UserLock::acquire()
             int err = getgrouplist(pw->pw_name, pw->pw_gid,
                 supplementaryGIDs.data(), &ngroups);
             if (err == -1)
-                throw Error(format("failed to get list of supplementary groups for ‘%1’") % pw->pw_name);
+                throw Error(format("failed to get list of supplementary groups for ‘%1%’") % pw->pw_name);
 
             supplementaryGIDs.resize(ngroups);