diff --git a/src/sentry/incidents/endpoints/serializers/workflow_engine_action.py b/src/sentry/incidents/endpoints/serializers/workflow_engine_action.py
index 38a85f3c68b7..37c6053c31bb 100644
--- a/src/sentry/incidents/endpoints/serializers/workflow_engine_action.py
+++ b/src/sentry/incidents/endpoints/serializers/workflow_engine_action.py
@@ -1,3 +1,4 @@
+import logging
 from collections.abc import Mapping
 from typing import Any, Sequence
 
@@ -18,6 +19,8 @@
 from sentry.users.services.user.model import RpcUser
 from sentry.workflow_engine.models import Action, ActionAlertRuleTriggerAction
 
+logger = logging.getLogger(__name__)
+
 
 class WorkflowEngineActionSerializer(Serializer[dict[str, Any]]):
     def get_attrs(
@@ -57,10 +60,16 @@ def serialize(
         type_value: int | None = ActionService.get_value(obj.type)
         target = attrs.get("target")
 
-        target_type: int = obj.config.get("target_type")
+        target_type: int | None = obj.config.get("target_type")
         target_identifier: str | None = obj.config.get("target_identifier")
         target_display: str | None = obj.config.get("target_display")
 
+        if target_type is None:
+            logger.warning(
+                "workflow_engine_action.missing_target_type",
+                extra={"action_id": obj.id, "action_type": obj.type},
+            )
+
         sentry_app_id = None
         sentry_app_config = None
         if obj.type == Action.Type.SENTRY_APP.value:
@@ -75,7 +84,9 @@ def serialize(
             ),
             "alertRuleTriggerId": str(alert_rule_trigger_id),
             "type": obj.type,
-            "targetType": ACTION_TARGET_TYPE_TO_STRING[ActionTarget(target_type)],
+            "targetType": ACTION_TARGET_TYPE_TO_STRING[ActionTarget(target_type)]
+            if target_type is not None
+            else None,
             "targetIdentifier": get_identifier_from_action(
                 type_value, str(target_identifier), target_display
             ),
diff --git a/tests/sentry/incidents/serializers/test_workflow_engine_action.py b/tests/sentry/incidents/serializers/test_workflow_engine_action.py
index fbce43b7f2cb..22a56013836f 100644
--- a/tests/sentry/incidents/serializers/test_workflow_engine_action.py
+++ b/tests/sentry/incidents/serializers/test_workflow_engine_action.py
@@ -188,6 +188,25 @@ def test_sentry_app_action(self) -> None:
         sentry_app_expected["desc"] = f"Send a notification via {sentry_app.name}"
         assert serialized_action == sentry_app_expected
 
+    def test_missing_target_type_does_not_raise(self) -> None:
+        """
+        Regression test for: ValueError: None is not a valid ActionTarget.
+        Actions whose config dict lacks a 'target_type' key (e.g. actions created
+        before the field was required) should serialize without raising.
+        """
+        # Mutate the migrated action's config to remove target_type, simulating
+        # the DB state that triggers the bug in production.
+        self.critical_action.config.pop("target_type", None)
+        self.critical_action.save()
+
+        serialized_action = serialize(
+            self.critical_action,
+            self.user,
+            WorkflowEngineActionSerializer(),
+            alert_rule_trigger_id=self.critical_trigger.id,
+        )
+        assert serialized_action["targetType"] is None
+
     def test_slack_action(self) -> None:
         self.integration = self.create_slack_integration(
             self.organization,