diff --git a/bin/kube-diagrams b/bin/kube-diagrams
index 59d8384..a213b92 100755
--- a/bin/kube-diagrams
+++ b/bin/kube-diagrams
@@ -172,11 +172,14 @@ class EdgesContext(list):
         def add_volumes(volumes):
             for volume in volumes:
                 if "configMap" in volume:
+                    configMap = volume["configMap"]
+                    configMap_id = f"{configMap.get('name')}/{self.namespace}/ConfigMap/v1"
+                    if configMap.get("optional") is True:
+                        if configMap_id not in resources:
+                            print(f"Warning: optional config map {configMap_id} undefined!")
+                            continue # skip it
                     self.append([
-                        "%s/%s/ConfigMap/v1" % (
-                            volume["configMap"]["name"],
-                            self.namespace
-                        ),
+                        configMap_id,
                         "REFERENCE"
                     ])
                 elif "secret" in volume:
@@ -185,11 +188,13 @@ class EdgesContext(list):
                     if secretName is None:
                         # Warning: jenkins chart uses 'name' instead of 'secretName'!
                         secretName = secret.get("name")
+                    secret_id = f"{secretName}/{self.namespace}/Secret/v1"
+                    if secret.get("optional") is True:
+                        if secret_id not in resources:
+                            print(f"Warning: optional secret {secret_id} undefined!")
+                            continue # skip it
                     self.append([
-                        "%s/%s/Secret/v1" % (
-                            secretName,
-                            self.namespace
-                        ),
+                        secret_id,
                         "REFERENCE"
                     ])
                 elif "persistentVolumeClaim" in volume:
@@ -225,11 +230,13 @@ class EdgesContext(list):
                     )
                 secretKeyRefName = query_path(env, "valueFrom.secretKeyRef.name")
                 if secretKeyRefName != None:
+                    secret_id = f"{secretKeyRefName}/{self.namespace}/Secret/v1"
+                    if query_path(env, "valueFrom.secretKeyRef.optional") is True:
+                        if secret_id not in resources:
+                            print(f"Warning: optional secret {secret_id} undefined!")
+                            continue # skip it
                     target_resources.add(
-                        "%s/%s/Secret/v1" % (
-                            secretKeyRefName,
-                            self.namespace
-                        )
+                        secret_id
                     )
         for target_resource in target_resources:
             self.append([target_resource, "REFERENCE"])
@@ -305,10 +312,13 @@ class EdgesContext(list):
 
     def add_subjects(self):
         for subject in query_path(self.resource, "subjects", []):
-            if subject.get("namespace"):
+            namespace = subject.get("namespace")
+            if namespace is None and subject["kind"] == "ServiceAccount":
+                namespace = get_namespace(self.resource)
+            if namespace != None:
                 rid = "%s/%s/%s/%s" % (
                     subject["name"],
-                    subject["namespace"],
+                    namespace,
                     subject["kind"],
                     query_path(subject, "apiGroup", "v1")
                 )
@@ -364,6 +374,9 @@ class EdgesContext(list):
                 ])
 
     def add_all_workload_resources(self, path, custom_workload_resources={}, edge_kind="SELECTOR"):
+        selector = query_path(self.resource, path)
+        if selector is None:
+            return
         build_in_workload_resources = {
             "Pod": "metadata.labels",
             "Deployment": "spec.template.metadata.labels",
@@ -555,6 +568,9 @@ with Diagram("", filename=args.output, show=False, direction="TB", outformat=arg
                     diagram_nodes[edge_rid] >> Edge(color="white", style="invisible") >> diagram_nodes[resource_id]
                 diagram_nodes[resource_id] >> Edge(**edge_config) >> diagram_nodes[edge_rid]
             except KeyError as ke:
+                if edge_rid in config["cluster-resources"]:
+                    print(f"Info: referenced {edge_rid} resource is provided by K8s clusters.")
+                    continue # skip this edge as the resource is provided by K8s clusters.
                 print("Error: %s resource not found!" % ke)
                 tmp = str(edge_rid).split("/")
                 if tmp[2] in ["ServiceAccount", "Service", "ConfigMap", "Secret"]: #TODO: avoid specific cases
diff --git a/bin/kube-diagrams.yaml b/bin/kube-diagrams.yaml
index e21b311..fdb9e65 100644
--- a/bin/kube-diagrams.yaml
+++ b/bin/kube-diagrams.yaml
@@ -315,3 +315,7 @@ nodes:
 #TODO: add other Kubernetes resource types.
 # PSP diagrams.k8s.others.PSP
 # Vol diagrams.k8s.storage.Vol, Volume (alias)
+
+cluster-resources:
+  - default/default/ServiceAccount/v1
+  - system:auth-delegator/ClusterRole/rbac.authorization.k8s.io/v1