diff --git a/pkgs/applications/science/math/sage/sage-src.nix b/pkgs/applications/science/math/sage/sage-src.nix
index 834590216870..51a83d70415b 100644
--- a/pkgs/applications/science/math/sage/sage-src.nix
+++ b/pkgs/applications/science/math/sage/sage-src.nix
@@ -50,6 +50,15 @@ stdenv.mkDerivation rec {
 
     # Fixes a potential race condition which can lead to transient doctest failures.
     ./patches/fix-ecl-race.patch
+
+    # Parallelize docubuild using subprocesses, fixing an isolation issue. See
+    # https://groups.google.com/forum/#!topic/sage-packaging/YGOm8tkADrE
+    (fetchpatch {
+      name = "sphinx-docbuild-subprocesses.patch";
+      url = "https://salsa.debian.org/science-team/sagemath/raw/8a215b17e6f791ddfae6df8ce6d01dfb89acb434/debian/patches/df-subprocess-sphinx.patch";
+      sha256 = "07p9i0fwjgapmfvmi436yn6v60p8pvmxqjc93wsssqgh5kd8qw3n";
+      stripLen = 1;
+    })
   ];
 
   # Patches needed because of package updates. We could just pin the versions of