diff --git a/pyproject.toml b/pyproject.toml
index 7b1c7e7f..cf135e50 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -88,7 +88,7 @@ dev-mypy = [
   "frequenz-client-dispatch[cli,dev-mkdocs,dev-noxfile,dev-pytest]",
   "grpc-stubs == 1.53.0.6",
   "types-protobuf == 6.30.2.20250703",
-  "types-python-dateutil == 2.9.0.20250516",
+  "types-python-dateutil == 2.9.0.20250708",
 ]
 dev-noxfile = ["nox == 2025.5.1", "frequenz-repo-config[lib] == 0.13.5"]
 dev-pylint = [
diff --git a/src/frequenz/client/dispatch/recurrence.py b/src/frequenz/client/dispatch/recurrence.py
index fef1993d..2c62bd22 100644
--- a/src/frequenz/client/dispatch/recurrence.py
+++ b/src/frequenz/client/dispatch/recurrence.py
@@ -197,8 +197,11 @@ def _as_rrule(self, start_time: datetime) -> rrule.rrule:
             The rrule object.
 
         Raises:
-            ValueError: If the interval is 0.
+            ValueError: If the interval is 0 or the frequency is UNSPECIFIED.
         """
+        if self.frequency == Frequency.UNSPECIFIED:
+            raise ValueError("Frequency must be specified")
+
         if self.interval == 0:
             raise ValueError("Interval must be greater than 0")
 
@@ -208,7 +211,10 @@ def _as_rrule(self, start_time: datetime) -> rrule.rrule:
             until = end.until
 
         rrule_obj = rrule.rrule(
-            freq=_RRULE_FREQ_MAP[self.frequency],
+            # Mypy expects a Literal for the `freq` argument, but it can't infer
+            # that the values from the `_RRULE_FREQ_MAP` dictionary are of the
+            # correct type.
+            freq=_RRULE_FREQ_MAP[self.frequency],  # type: ignore[arg-type]
             dtstart=start_time,
             count=count,
             until=until,