diff --git a/pyproject.toml b/pyproject.toml
index 96978cf..b0b67d1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,7 +28,7 @@ classifiers = [
     "Topic :: Text Processing :: Markup :: Markdown",
 ]
 dependencies = [
-    "PySide6>=6.5.0",
+    "PySide6>=6.5.0,!=6.12.0",
     "markdown>=3.4.0",
 ]
 
diff --git a/requirements-build.txt b/requirements-build.txt
index 8ca9689..0c83945 100644
--- a/requirements-build.txt
+++ b/requirements-build.txt
@@ -1,3 +1,3 @@
 pyinstaller==6.14.2
-PySide6>=6.10,<7
+PySide6>=6.10,<7,!=6.12.0
 Markdown>=3.10,<4
diff --git a/requirements.txt b/requirements.txt
index d859bed..10e0643 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,3 @@
-PySide6>=6.10,<7
+# 6.12.0 corrupts None refcounts in virtual highlighter callbacks.
+PySide6>=6.10,<7,!=6.12.0
 Markdown>=3.10,<4