diff --git a/pkgs/by-name/er/eresi/package.nix b/pkgs/by-name/er/eresi/package.nix
index 97b7e50c295e..d1b8470c2d45 100644
--- a/pkgs/by-name/er/eresi/package.nix
+++ b/pkgs/by-name/er/eresi/package.nix
@@ -40,6 +40,9 @@ stdenv.mkDerivation (finalAttrs: {
     })
   ];
 
+  # K&R-style function-pointer declarations break under gcc 15's C23 default.
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   postPatch = ''
     # Two occurences of fprintf() with only two arguments, which should really
     # be fputs().