diff --git a/.circleci/config.yml b/.circleci/config.yml
index 7db6185c..24a6fe62 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -15,7 +15,7 @@ version: 2
     - run:
         name: Run the tests in a container
         command: |
-          docker run -e CI=true --rm -v `pwd`:`pwd` -w `pwd` "pqclean/ci-container:$ARCH" /bin/bash -c "
+          docker run -e CI=true -e PQCLEAN_ONLY_DIFF=1 --rm -v `pwd`:`pwd` -w `pwd` "pqclean/ci-container:$ARCH" /bin/bash -c "
           uname -a &&
           export CC=${CC} &&
           pip3 install -r requirements.txt &&
@@ -35,6 +35,7 @@ version: 2
         name: Run tests
         command: |
           export CC=${CC}
+          export PQCLEAN_ONLY_DIFF=1
           pip3 install -r requirements.txt &&
           cd test && python3 -m nose --rednose --verbose
 
diff --git a/.travis.yml b/.travis.yml
index 6d7d3780..6b72ac2c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,8 @@ matrix:
         - pip3 install -r requirements.txt
       script:
         - "cd test && python3 -m nose --rednose --verbose"
+      env:
+        PQCLEAN_ONLY_DIFF: 1
       addons:
         homebrew:
           packages:
@@ -21,6 +23,8 @@ matrix:
           packages:
             - astyle
             - gcc@8
+      env:
+        PQCLEAN_ONLY_DIFF: 1
       before_install:
         - pip3 install -r requirements.txt
         - brew link gcc
diff --git a/appveyor.yml b/appveyor.yml
index 956ad13d..e9b9464e 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -6,6 +6,7 @@ build:
   verbosity: minimal
 
 environment:
+  PQCLEAN_ONLY_DIFF: 1
   matrix:
     - BITS: 64
     - BITS: 32
diff --git a/test/helpers.py b/test/helpers.py
index ea1639af..2fcd3f03 100644
--- a/test/helpers.py
+++ b/test/helpers.py
@@ -142,7 +142,7 @@ def permit_test(testname, thing, **args):
             return False
 
     if 'PQCLEAN_ONLY_DIFF' in os.environ:
-        if shutil.which('git') != None:
+        if shutil.which('git') is not None:
             # if we're on a non-master branch, and the only changes are in schemes, 
             # only run tests on those schemes
             branch_result = subprocess.run(