diff --git a/Lib/fontTools/ttLib/tables/otData.py b/Lib/fontTools/ttLib/tables/otData.py
index 3a01f5934..f538aeb48 100644
--- a/Lib/fontTools/ttLib/tables/otData.py
+++ b/Lib/fontTools/ttLib/tables/otData.py
@@ -385,7 +385,7 @@ otData = [
             (
                 "DeltaValue",
                 "DeltaValue",
-                "",
+                None,
                 "DeltaFormat in (1,2,3)",
                 "Array of compressed data",
             ),