diff --git a/synapse/util/stringutils.py b/synapse/util/stringutils.py
index 11246d95e9..27a363d7e5 100644
--- a/synapse/util/stringutils.py
+++ b/synapse/util/stringutils.py
@@ -250,4 +250,8 @@ def base62_encode(num: int, minwidth: int = 1) -> str:
 
 
 def non_null_str_or_none(val: Any) -> Optional[str]:
+    """Check that the arg is a string containing no null (U+0000) codepoints.
+
+    If so, returns the given string unmodified; otherwise, returns None.
+    """
     return val if isinstance(val, str) and "\u0000" not in val else None