diff --git a/tangostationcontrol/tangostationcontrol/devices/sdp/beamlet.py b/tangostationcontrol/tangostationcontrol/devices/sdp/beamlet.py
index 35d484139e2dcc0462211bbb6a4542062e06ff01..717d2d9bb38e54dbb0eed63548955498d06cd010 100644
--- a/tangostationcontrol/tangostationcontrol/devices/sdp/beamlet.py
+++ b/tangostationcontrol/tangostationcontrol/devices/sdp/beamlet.py
@@ -152,7 +152,7 @@ class Beamlet(opcua_device):
         real = numpy.array(unit * numpy.sin(phases), dtype=numpy.short)
         imag = numpy.array(unit * numpy.cos(phases), dtype=numpy.short)
         # join 16 bits of imaginary part (MSB) with 16 bits of real part (LSB)
-        bf_weights = numpy.array( numpy.frombuffer( b''.join(imag,real), dtype=numpy.uint32 ) )
+        bf_weights = numpy.array( numpy.frombuffer( b''.join([imag,real]), dtype=numpy.uint32 ) )
         
         return bf_weights