diff --git a/pkgs/by-name/mo/monosat/package.nix b/pkgs/by-name/mo/monosat/package.nix
index 7d5bea0a95d9..4ae627accfb3 100644
--- a/pkgs/by-name/mo/monosat/package.nix
+++ b/pkgs/by-name/mo/monosat/package.nix
@@ -116,7 +116,11 @@ let
       # the sourceRoot if it weren't for the patch.
       postPatch =
         commonPostPatch
+        # cython 3.1 dropped the python 2 `long` builtin
         + ''
+          substituteInPlace src/monosat/api/python/monosat/monosat_p.pyx \
+            --replace-fail '(int, long)' 'int' \
+            --replace-fail '(int,long)' 'int'
           cd src/monosat/api/python
         ''
         +