diff --git a/.changeset/brave-radios-live.md b/.changeset/brave-radios-live.md
new file mode 100644
index 00000000000..8f24a2324b1
--- /dev/null
+++ b/.changeset/brave-radios-live.md
@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: Prevent crash when flowchart node shape is undefined
diff --git a/cypress/integration/rendering/flowchart/flowchart-v2.spec.js b/cypress/integration/rendering/flowchart/flowchart-v2.spec.js
index 4cb54e5087a..9adb1f4be39 100644
--- a/cypress/integration/rendering/flowchart/flowchart-v2.spec.js
+++ b/cypress/integration/rendering/flowchart/flowchart-v2.spec.js
@@ -1290,6 +1290,37 @@ class link myClass
     );
   });
 
+  it('V2 - 18: should render nested subgraphs with edge from cluster containing extractable subgraph', () => {
+    imgSnapshotTest(
+      `flowchart TB
+    subgraph asub
+        aa
+    end
+   a
+    subgraph bsub
+        subgraph csub
+            subgraph dsub
+                da
+            end
+        end
+        subgraph esub
+            subgraph fsub
+                %%comment out subgraph below
+                subgraph gsub
+                    ga
+                    abc
+                    abcd
+                end
+            end
+        end
+    end
+    bsub-->a
+    da-->a
+      `,
+      {}
+    );
+  });
+
   describe('Edge label autowrapping', () => {
     it('should wrap edge labels', () => {
       imgSnapshotTest(
@@ -1308,7 +1339,7 @@ config: ${JSON.stringify({ markdownAutoWrap, htmlLabels })}
 title: Testing with ${JSON.stringify({ markdownAutoWrap, htmlLabels })}
 ---
 flowchart TD
-    A["This is a really long line of plain text that will autowrap and support \\n newlines too."]    
+    A["This is a really long line of plain text that will autowrap and support \\n newlines too."]
     B["\`This is a really long line of **markdown** text that will autowrap, unless markdownAutoWrap:false is set.\`"]
     A -- "Plain text **labels** in flowcharts will autowrap,like node labels. \\n Newline characters work too." --> B
     B -- "\`**Markdown** edge labels will autowrap, unless markdownAutoWrap: false is set\`" --> C
diff --git a/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js b/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
index f0e5cd5ed9b..bab1c6c479f 100644
--- a/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
+++ b/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
@@ -247,6 +247,22 @@ export const adjustClustersAndEdges = (graph, depth) => {
     if (parent !== id && clusterDb.has(parent) && !clusterDb.get(parent).externalConnections) {
       clusterDb.get(id).id = parent;
     }
+    // When this cluster has a direct outgoing edge AND its current anchor sits inside
+    // a sibling subgraph that will be extracted (collapsed into a clusterNode), the
+    // anchor will disappear by render time and the edge endpoint becomes undefined.
+    // Re-anchor onto a node that survives extraction.
+    const hasDirectOutgoingEdge = graph.edges().some((edge) => edge.v === id);
+    if (
+      nonClusterChild &&
+      clusterDb.get(id)?.externalConnections &&
+      hasDirectOutgoingEdge &&
+      isNodeInExtractableCluster(graph, nonClusterChild, id)
+    ) {
+      const safeAnchor = findSafeAnchorNode(graph, id, graph.parent(nonClusterChild));
+      if (safeAnchor) {
+        clusterDb.get(id).id = safeAnchor;
+      }
+    }
   }
 
   graph.edges().forEach(function (e) {
@@ -411,3 +427,43 @@ const sorter = (graph, nodes) => {
 };
 
 export const sortNodesByHierarchy = (graph) => sorter(graph, graph.children());
+
+/** Checks if a node is inside a cluster that will be extracted (has no external connections). */
+const isNodeInExtractableCluster = (graph, node, rootId) => {
+  let parent = graph.parent(node);
+
+  while (parent && parent !== rootId) {
+    const cluster = clusterDb.get(parent);
+    if (cluster && !cluster.externalConnections) {
+      return true;
+    }
+    parent = graph.parent(parent);
+  }
+
+  return false;
+};
+
+/** Finds an alternative anchor node for a cluster that is not inside an extractable cluster. */
+const findSafeAnchorNode = (graph, clusterId, excludedCluster) => {
+  const children = graph.children(clusterId) ?? [];
+
+  for (const child of children) {
+    if (child === excludedCluster || isDescendant(child, excludedCluster)) {
+      continue;
+    }
+
+    // findNonClusterChild returns the leaf itself when child is a leaf, or drills
+    // into a subgraph to find a non-cluster descendant. A returned leaf sibling is
+    // a perfectly valid anchor — only skip when the lookup found nothing usable.
+    const candidate = findNonClusterChild(child, graph, clusterId);
+    if (!candidate) {
+      continue;
+    }
+
+    if (!isNodeInExtractableCluster(graph, candidate, clusterId)) {
+      return candidate;
+    }
+  }
+
+  return null;
+};