diff --git a/RELEASE_NOTES.rst b/RELEASE_NOTES.rst
index 89c8dd740..2fd2426b7 100644
--- a/RELEASE_NOTES.rst
+++ b/RELEASE_NOTES.rst
@@ -9,6 +9,8 @@ unreleased
 * The CPU speed in the pipistrello gateware has been reduced from 83 1/3 MHz to
   75 MHz. This will reduce the achievable sustained pulse rate and latency
   accordingly. ISE was intermittently failing to meet timing (#341).
+* set_dataset in broadcast mode no longer returns a Notifier. Mutating datasets
+  should be done with mutate_dataset instead (#345).
 
 
 1.0rc1
diff --git a/artiq/language/environment.py b/artiq/language/environment.py
index b73ded63c..2654b4702 100644
--- a/artiq/language/environment.py
+++ b/artiq/language/environment.py
@@ -221,13 +221,11 @@ class HasEnvironment:
                     broadcast=False, persist=False, save=True):
         """Sets the contents and handling modes of a dataset.
 
-        If the dataset is broadcasted, it must be PYON-serializable.
-        If the dataset is saved, it must be a scalar (``bool``, ``int``,
-        ``float`` or NumPy scalar) or a NumPy array.
+        Datasets must be scalars (``bool``, ``int``, ``float`` or NumPy scalar)
+        or NumPy arrays.
 
         :param broadcast: the data is sent in real-time to the master, which
-            dispatches it. Returns a Notifier that can be used to mutate the
-            dataset.
+            dispatches it.
         :param persist: the master should store the data on-disk. Implies
             broadcast.
         :param save: the data is saved into the local storage of the current
@@ -238,7 +236,19 @@ class HasEnvironment:
             return
         if self.__dataset_mgr is None:
             raise ValueError("Dataset manager not present")
-        return self.__dataset_mgr.set(key, value, broadcast, persist, save)
+        self.__dataset_mgr.set(key, value, broadcast, persist, save)
+
+    def mutate_dataset(self, key, index, value):
+        """Mutate an existing dataset at the given index (e.g. set a value at
+        a given position in a NumPy array)
+
+        If the dataset was created in broadcast mode, the modification is
+        immediately transmitted."""
+        if self.__parent is not None:
+            self.__parent.mutate_dataset(key, index, value)
+        if self.__dataset_mgr is None:
+            raise ValueError("Dataset manager not present")
+        self.__dataset_mgr.mutate(key, index, value)
 
     def get_dataset(self, key, default=NoDefault):
         """Returns the contents of a dataset.
diff --git a/artiq/master/worker_db.py b/artiq/master/worker_db.py
index c68c24f6f..eadc0a0b9 100644
--- a/artiq/master/worker_db.py
+++ b/artiq/master/worker_db.py
@@ -228,20 +228,26 @@ class DatasetManager:
     def set(self, key, value, broadcast=False, persist=False, save=True):
         if persist:
             broadcast = True
-        r = None
         if broadcast:
-            self.broadcast[key] = (persist, value)
-            r = self.broadcast[key][1]
+            self.broadcast[key] = persist, value
         if save:
             self.local[key] = value
-        return r
+
+    def mutate(self, key, index, value):
+        target = None
+        if key in self.local:
+            target = self.local[key]
+        if key in self.broadcast.read:
+            target = self.broadcast[key][1]
+        if target is None:
+            raise KeyError("Cannot mutate non-existing dataset")
+        target[index] = value
 
     def get(self, key):
-        try:
+        if key in self.local:
             return self.local[key]
-        except KeyError:
-            pass
-        return self.ddb.get(key)
+        else:
+            return self.ddb.get(key)
 
     def write_hdf5(self, f):
         result_dict_to_hdf5(f, self.local)
diff --git a/examples/master/repository/flopping_f_simulation.py b/examples/master/repository/flopping_f_simulation.py
index 6f60557ac..0315ef2d8 100644
--- a/examples/master/repository/flopping_f_simulation.py
+++ b/examples/master/repository/flopping_f_simulation.py
@@ -38,19 +38,19 @@ class FloppingF(EnvExperiment):
 
     def run(self):
         l = len(self.frequency_scan)
-        frequency = self.set_dataset("flopping_f_frequency",
-                                     np.full(l, np.nan),
-                                     broadcast=True, save=False)
-        brightness = self.set_dataset("flopping_f_brightness",
-                                      np.full(l, np.nan),
-                                     broadcast=True)
+        self.set_dataset("flopping_f_frequency",
+                         np.full(l, np.nan),
+                         broadcast=True, save=False)
+        self.set_dataset("flopping_f_brightness",
+                         np.full(l, np.nan),
+                         broadcast=True)
         self.set_dataset("flopping_f_fit", np.full(l, np.nan),
                          broadcast=True, save=False)
 
         for i, f in enumerate(self.frequency_scan):
             m_brightness = model(f, self.F0) + self.noise_amplitude*random.random()
-            frequency[i] = f
-            brightness[i] = m_brightness
+            self.mutate_dataset("flopping_f_frequency", i, f)
+            self.mutate_dataset("flopping_f_brightness", i, m_brightness)
             time.sleep(0.1)
         self.scheduler.submit(self.scheduler.pipeline_name, self.scheduler.expid,
                               self.scheduler.priority, time.time() + 20, False)