diff --git a/src/main/python/ttconv/model.py b/src/main/python/ttconv/model.py
index d2f96b27..1c9da315 100644
--- a/src/main/python/ttconv/model.py
+++ b/src/main/python/ttconv/model.py
@@ -597,7 +597,7 @@ def push_children(self, children: typing.Iterable[ContentElement]):
 
     ts = [type(x) for x in children]
 
-    if ts not in [[Rb, Rt], [Rb, Rp, Rt, Rp], [Rbc, Rtc], [Rbc, Rtc, Rtc]]:
+    if ts not in [[], [Rb, Rt], [Rb, Rp, Rt, Rp], [Rbc, Rtc], [Rbc, Rtc, Rtc]]:
       raise ValueError("Children of ruby do not conform to requirements")
 
     for child in children: