diff --git a/python/cudf/cudf/core/udf/utils.py b/python/cudf/cudf/core/udf/utils.py
index b54b7d3bcf6..a98ee40274e 100644
--- a/python/cudf/cudf/core/udf/utils.py
+++ b/python/cudf/cudf/core/udf/utils.py
@@ -144,6 +144,7 @@ def _construct_signature(frame, return_type, args):
 
 @cuda.jit(device=True)
 def _mask_get(mask, pos):
+    """Return the validity of mask[pos] as a word."""
     return (mask[pos // MASK_BITSIZE] >> (pos % MASK_BITSIZE)) & 1