diff --git a/.github/workflows/test-and-release.yml b/.github/workflows/test-and-release.yml
index ebf768c..9ea622c 100644
--- a/.github/workflows/test-and-release.yml
+++ b/.github/workflows/test-and-release.yml
@@ -1,5 +1,5 @@
-name: 'Test and Release Yakut'
-on: push
+name: 'Test and release Yakut'
+on: [ push, pull_request ]
 
 # Ensures that only one workflow is running at a time
 concurrency:
@@ -9,6 +9,8 @@ concurrency:
 jobs:
   yakut-test:
     name: Test Yakut
+    # https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=edited#pull_request
+    if: (github.event_name == 'push') || github.event.pull_request.head.repo.fork
     strategy:
       fail-fast: false
       matrix:
@@ -71,7 +73,9 @@ jobs:
   yakut-release:
     name: Release Yakut
     runs-on: ubuntu-latest
-    if: contains(github.event.head_commit.message, '#release') || contains(github.ref, '/main')
+    if: >
+      (github.event_name == 'push') &&
+      (contains(github.event.head_commit.message, '#release') || contains(github.ref, '/main'))
     needs: yakut-test
     steps:
       - name: Check out
diff --git a/yakut/VERSION b/yakut/VERSION
index 9beb74d..288adf5 100644
--- a/yakut/VERSION
+++ b/yakut/VERSION
@@ -1 +1 @@
-0.13.2
+0.13.3
diff --git a/yakut/cmd/monitor/_model.py b/yakut/cmd/monitor/_model.py
index 8add024..eaa28e9 100644
--- a/yakut/cmd/monitor/_model.py
+++ b/yakut/cmd/monitor/_model.py
@@ -191,8 +191,8 @@ def __repr__(self) -> str:
 def expand_subjects(m: uavcan.node.port.SubjectIDList_1) -> AbstractSet[int]:
     if m.sparse_list is not None:
         return frozenset(int(x.value) for x in m.sparse_list)
-    if m.mask:
-        return expand_mask(m.mask)
+    if m.mask is not None and m.mask.any():
+            return expand_mask(m.mask)
     if m.total:
         return _COMPLETE_SUBJECT_SET
     assert False