diff --git a/src/nix/main.cc b/src/nix/main.cc
index dcf351cf4..5454f4b34 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -104,7 +104,7 @@ 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);