diff --git a/nimpy.nim b/nimpy.nim
index 2563db05a..21d68c9aa 100644
--- a/nimpy.nim
+++ b/nimpy.nim
@@ -843,6 +843,8 @@ proc nimJsonToPy(node: JsonNode): PPyObject =
       decRef vv
       if ret != 0:
         cannotSerializeErr(k)
+  else:
+    doAssert false, "not yet implemented, pending https://github.com/nim-lang/Nim/pull/15545"
 
 # Enum handling
 proc nimpyEnumConvert*[T](o: T): int=