diff --git a/pyproject.toml b/pyproject.toml
index 922c159..dbedb34 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -56,7 +56,7 @@ classifiers = [
     "Topic :: Utilities"
 ]
 dependencies = [
-    "PySide6>=6.5.0",
+    "PySide6>=6.5.0,!=6.12.0",
 ]
 
 [project.optional-dependencies]
diff --git a/requirements.txt b/requirements.txt
index e18399b..ce188a9 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,3 @@
 # CodeBox Dependencies
-PySide6>=6.5.0
+# 6.12.0 corrupts None refcounts in isolated Qt callbacks (diagnostic PR #4).
+PySide6>=6.5.0,!=6.12.0