diff --git a/test/functional/test_framework/mininode.py b/test/functional/test_framework/mininode.py
--- a/test/functional/test_framework/mininode.py
+++ b/test/functional/test_framework/mininode.py
@@ -193,12 +193,12 @@
                     raise ValueError(
                         "got garbage {}".format(repr(self.recvbuf)))
                 if len(self.recvbuf) < 4 + 12 + 4 + 4:
-                    return
+                    return None
                 command = self.recvbuf[4:4+12].split(b"\x00", 1)[0]
                 msglen = struct.unpack("<i", self.recvbuf[4+12:4+12+4])[0]
                 checksum = self.recvbuf[4+12+4:4+12+4+4]
                 if len(self.recvbuf) < 4 + 12 + 4 + 4 + msglen:
-                    return
+                    return None
                 msg = self.recvbuf[4+12+4+4:4+12+4+4+msglen]
                 h = sha256(sha256(msg))
                 if checksum != h[:4]: