diff --git a/src/nix/main.cc b/src/nix/main.cc
index 01b0866f2..3ec5f48d5 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -57,7 +57,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             "--help-config' for a list of configuration settings.\n";
     }
 
-    void printHelp(const string & programName, std::ostream & out)
+    void printHelp(const string & programName, std::ostream & out) override
     {
         MultiCommand::printHelp(programName, out);