diff --git a/tasks/sud.py b/tasks/sud.py
index 0f7db45..c280228 100644
--- a/tasks/sud.py
+++ b/tasks/sud.py
@@ -66,7 +66,10 @@ class SudTask(ATask):
 		# its whole duration (decided once, here, rather than re-checked
 		# every tick) instead of relying on the controller's own FSM to
 		# notice and idle out.
-		cooling = ramping and ramp is not None and ramp['temp'] < self.tc.get_theta_ist() - TEMP_REACHED_TOLERANCE
+		# bool(...): get_theta_ist() can be a numpy.float64 (Pot's transport
+		# delay line is a numpy array internally) - the '<' comparison would
+		# then yield numpy.bool_, which json.dumps() can't serialize.
+		cooling = bool(ramping and ramp is not None and ramp['temp'] < self.tc.get_theta_ist() - TEMP_REACHED_TOLERANCE)
 		self.tc.set_cooling(cooling)
 
 		if step is not None: