diff --git a/default.nix b/default.nix
index 4c9ed1cc9..b6918ecb7 100644
--- a/default.nix
+++ b/default.nix
@@ -46,7 +46,21 @@ mach-nix.buildPythonPackage {
   providers = {
     # Through zfec 1.5.5 the wheel has an incorrect runtime dependency
     # declared on argparse, not available for recent versions of Python 3.
-    # Force mach-nix to use the sdist instead, side-stepping this issue.
+    # Force mach-nix to use the sdist instead.  This allows us to apply a
+    # patch that removes the offending declaration.
     zfec = "sdist";
   };
+
+  # Define certain overrides to the way Python dependencies are built.
+  _ = {
+    # Apply the argparse declaration fix to zfec sdist.
+    zfec.patches = with pkgs; [
+      (fetchpatch {
+        name = "fix-argparse.patch";
+        url = "https://github.com/tahoe-lafs/zfec/commit/c3e736a72cccf44b8e1fb7d6c276400204c6bc1e.patch";
+        sha256 = "1md9i2fx1ya7mgcj9j01z58hs3q9pj4ch5is5b5kq4v86cf6x33x";
+      })
+    ];
+  };
+
 }