diff --git a/pkgs/by-name/ny/nylon/package.nix b/pkgs/by-name/ny/nylon/package.nix
index e733c231fc40..3e953c083e74 100644
--- a/pkgs/by-name/ny/nylon/package.nix
+++ b/pkgs/by-name/ny/nylon/package.nix
@@ -29,6 +29,9 @@ stdenv.mkDerivation (finalAttrs: {
 
   buildInputs = [ libevent ];
 
+  # K&R-style function pointer declarations break under gcc 15's C23 default.
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   meta = {
     homepage = "http://monkey.org/~marius/nylon";
     description = "Proxy server, supporting SOCKS 4 and 5, as well as a mirror mode";