diff --git a/Lib/fontTools/ttLib/tables/otConverters.py b/Lib/fontTools/ttLib/tables/otConverters.py
index afe4e538f4..a2f672567e 100644
--- a/Lib/fontTools/ttLib/tables/otConverters.py
+++ b/Lib/fontTools/ttLib/tables/otConverters.py
@@ -153,6 +153,8 @@ def __init__(self, name, repeat, aux, tableClass=None, *, description=""):
         self.name = name
         self.repeat = repeat
         self.aux = aux
+        if self.aux and not self.repeat:
+            self.aux = compile(self.aux, "<string>", "eval")
         self.tableClass = tableClass
         self.isCount = name.endswith("Count") or name in [
             "DesignAxisRecordSize",