diff --git a/scripts-dev/mypy_synapse_plugin.py b/scripts-dev/mypy_synapse_plugin.py
index 7fe4d6cd86..ac1fee7f67 100644
--- a/scripts-dev/mypy_synapse_plugin.py
+++ b/scripts-dev/mypy_synapse_plugin.py
@@ -45,6 +45,7 @@ from mypy.types import (
     AnyType,
     CallableType,
     Instance,
+    LiteralType,
     NoneType,
     Options,
     TupleType,
@@ -813,6 +814,10 @@ def is_cacheable(
     if isinstance(rt, AnyType):
         return True, ("may be mutable" if verbose else None)
 
+    elif isinstance(rt, LiteralType):
+        # Literal[True] etc
+        return True, None
+
     elif isinstance(rt, Instance):
         if (
             rt.type.fullname in IMMUTABLE_VALUE_TYPES