diff --git a/src/vpp-api/python/vpp_papi.py b/src/vpp-api/python/vpp_papi.py
index 35edd49..5fa82a3 100644
--- a/src/vpp-api/python/vpp_papi.py
+++ b/src/vpp-api/python/vpp_papi.py
@@ -639,7 +639,7 @@
         """Send a binary-packed message to VPP."""
         if not self.connected:
             raise IOError(1, 'Not connected')
-        return vpp_api.vac_write(str(buf), len(buf))
+        return vpp_api.vac_write(bytes(buf), len(buf))
 
     def _read(self):
         if not self.connected: