diff --git a/pkgs/tools/networking/unbound/default.nix b/pkgs/tools/networking/unbound/default.nix
index 393756a9b2ab..bd994c2640f9 100644
--- a/pkgs/tools/networking/unbound/default.nix
+++ b/pkgs/tools/networking/unbound/default.nix
@@ -82,7 +82,7 @@ stdenv.mkDerivation rec {
     "--with-libhiredis=${hiredis}"
   ];
 
-  PROTOC_C = if withDNSTAP then "${protobufc}/bin/protoc-c" else null;
+  PROTOC_C = lib.optionalString withDNSTAP "${protobufc}/bin/protoc-c";
 
   # Remove references to compile-time dependencies that are included in the configure flags
   postConfigure = let