diff --git a/nix/build-all.py b/nix/build-all.py
index 2cb785883f..f008b8f940 100644
--- a/nix/build-all.py
+++ b/nix/build-all.py
@@ -1237,7 +1237,7 @@ if "cgal" in targets:
             f"-DMPFR_INCLUDE_DIR={DEPS_DIR}/install/mpfr-{MPFR_VERSION}/include",
             f"-DBoost_INCLUDE_DIR={DEPS_DIR}/install/boost-{BOOST_VERSION}",
             f"-DCMAKE_INSTALL_PREFIX={DEPS_DIR}/install/cgal-{CGAL_VERSION}/",
-            f"-WITH_GMPXX=Off",
+            f"-DWITH_GMPXX=Off",
             f"-DCGAL_HEADER_ONLY=On",
             f"-DBUILD_SHARED_LIBS=Off",
         ],