diff --git a/tests/extmod/ujson_dump_iobase.py b/tests/extmod/ujson_dump_iobase.py
index d30d1b561e..51d507cd1a 100644
--- a/tests/extmod/ujson_dump_iobase.py
+++ b/tests/extmod/ujson_dump_iobase.py
@@ -24,6 +24,7 @@ class S(io.IOBase):
             # uPy passes a bytearray, CPython passes a str
             buf = str(buf, 'ascii')
         self.buf += buf
+        return len(buf)
 
 
 # dump to the user stream