diff --git a/pkgs/development/python-modules/generic/default.nix b/pkgs/development/python-modules/generic/default.nix
index 15f369a44f2..1fdbd4ffc0b 100644
--- a/pkgs/development/python-modules/generic/default.nix
+++ b/pkgs/development/python-modules/generic/default.nix
@@ -42,6 +42,9 @@
 # generated binaries.
 , makeWrapperArgs ? []
 
+# Additional flags to pass to "pip install".
+, installFlags ? []
+
 , ... } @ attrs:
 
 
@@ -95,7 +98,7 @@ python.stdenv.mkDerivation (builtins.removeAttrs attrs ["disabled" "doCheck"] //
     export PYTHONPATH="$out/${python.sitePackages}:$PYTHONPATH"
 
     pushd dist
-    ${bootstrapped-pip}/bin/pip install *.whl --no-index --prefix=$out --no-cache
+    ${bootstrapped-pip}/bin/pip install *.whl --no-index --prefix=$out --no-cache ${toString installFlags}
     popd
 
     runHook postInstall
diff --git a/pkgs/top-level/python-packages.nix b/pkgs/top-level/python-packages.nix
index f925e688933..df742a5c84c 100644
--- a/pkgs/top-level/python-packages.nix
+++ b/pkgs/top-level/python-packages.nix
@@ -15067,6 +15067,10 @@ in modules // {
       sha256 = "46f4bd0d8dfd51125a554568d646fe4200a3c2c6c36b9f2d06d2212148439521";
     };
 
+    # pip detects that we already have bootstrapped_pip "installed", so we need
+    # to force it a little.
+    installFlags = [ "--ignore-installed" ];
+
     buildInputs = with self; [ mock scripttest virtualenv pytest ];
   };
 
@@ -15492,7 +15496,7 @@ in modules // {
       ${python.executable} setup.py test
     '';
 
-    installFlags = optional (versionAtLeast protobuf.version "2.6.0") "--cpp_implementation";
+    installFlags = optional (versionAtLeast protobuf.version "2.6.0") "--install-option='--cpp_implementation'";
 
     doCheck = true;