diff --git a/.changeset/thick-elephants-search.md b/.changeset/thick-elephants-search.md
new file mode 100644
index 0000000000..5e29c42d66
--- /dev/null
+++ b/.changeset/thick-elephants-search.md
@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: error `mermaid.parse` on an invalid shape, so that it matches the errors thrown by `mermaid.render`
diff --git a/packages/mermaid/src/diagrams/flowchart/flowDb.ts b/packages/mermaid/src/diagrams/flowchart/flowDb.ts
index 8d8245e677..9e3f64a6ce 100644
--- a/packages/mermaid/src/diagrams/flowchart/flowDb.ts
+++ b/packages/mermaid/src/diagrams/flowchart/flowDb.ts
@@ -2,6 +2,7 @@ import { select } from 'd3';
 import utils, { getEdgeId } from '../../utils.js';
 import { getConfig, defaultConfig } from '../../diagram-api/diagramAPI.js';
 import common from '../common/common.js';
+import { isValidShape, type ShapeID } from '../../rendering-util/rendering-elements/shapes.js';
 import type { Node, Edge } from '../../rendering-util/types.js';
 import { log } from '../../logger.js';
 import * as yaml from 'js-yaml';
@@ -14,7 +15,15 @@ import {
   setDiagramTitle,
   getDiagramTitle,
 } from '../common/commonDb.js';
-import type { FlowVertex, FlowClass, FlowSubGraph, FlowText, FlowEdge, FlowLink } from './types.js';
+import type {
+  FlowVertex,
+  FlowClass,
+  FlowSubGraph,
+  FlowText,
+  FlowEdge,
+  FlowLink,
+  FlowVertexTypeParam,
+} from './types.js';
 import type { NodeMetaData } from '../../types.js';
 
 const MERMAID_DOM_ID_PREFIX = 'flowchart-';
@@ -53,12 +62,11 @@ export const lookUpDomId = function (id: string) {
 
 /**
  * Function called by parser when a node definition has been found
- *
  */
 export const addVertex = function (
   id: string,
   textObj: FlowText,
-  type: 'group',
+  type: FlowVertexTypeParam,
   style: string[],
   classes: string[],
   dir: string,
@@ -133,14 +141,15 @@ export const addVertex = function (
     }
     // console.log('yamlData', yamlData);
     const doc = yaml.load(yamlData, { schema: yaml.JSON_SCHEMA }) as NodeMetaData;
-    if (doc.shape && (doc.shape !== doc.shape.toLowerCase() || doc.shape.includes('_'))) {
-      throw new Error(`No such shape: ${doc.shape}. Shape names should be lowercase.`);
-    }
-
-    // console.log('yamlData doc', doc);
-    if (doc?.shape) {
+    if (doc.shape) {
+      if (doc.shape !== doc.shape.toLowerCase() || doc.shape.includes('_')) {
+        throw new Error(`No such shape: ${doc.shape}. Shape names should be lowercase.`);
+      } else if (!isValidShape(doc.shape)) {
+        throw new Error(`No such shape: ${doc.shape}.`);
+      }
       vertex.type = doc?.shape;
     }
+
     if (doc?.label) {
       vertex.text = doc?.label;
     }
@@ -816,7 +825,7 @@ export const lex = {
   firstGraph,
 };
 
-const getTypeFromVertex = (vertex: FlowVertex) => {
+const getTypeFromVertex = (vertex: FlowVertex): ShapeID => {
   if (vertex.img) {
     return 'imageSquare';
   }
@@ -832,14 +841,18 @@ const getTypeFromVertex = (vertex: FlowVertex) => {
     }
     return 'icon';
   }
-  if (vertex.type === 'square') {
-    return 'squareRect';
-  }
-  if (vertex.type === 'round') {
-    return 'roundedRect';
+  switch (vertex.type) {
+    case 'square':
+    case undefined:
+      return 'squareRect';
+    case 'round':
+      return 'roundedRect';
+    case 'ellipse':
+      // @ts-expect-error -- Ellipses are broken, see https://github.com/mermaid-js/mermaid/issues/5976
+      return 'ellipse';
+    default:
+      return vertex.type;
   }
-
-  return vertex.type ?? 'squareRect';
 };
 
 const findNode = (nodes: Node[], id: string) => nodes.find((node) => node.id === id);
diff --git a/packages/mermaid/src/diagrams/flowchart/parser/flow-node-data.spec.js b/packages/mermaid/src/diagrams/flowchart/parser/flow-node-data.spec.js
index 42e3bbbb4e..1669cfada0 100644
--- a/packages/mermaid/src/diagrams/flowchart/parser/flow-node-data.spec.js
+++ b/packages/mermaid/src/diagrams/flowchart/parser/flow-node-data.spec.js
@@ -197,6 +197,21 @@ describe('when parsing directions', function () {
     expect(data4Layout.nodes[0].shape).toEqual('squareRect');
     expect(data4Layout.nodes[0].label).toEqual('This is }');
   });
+  it('should error on non-existent shape', function () {
+    expect(() => {
+      flow.parser.parse(`flowchart TB
+      A@{ shape: this-shape-does-not-exist }
+      `);
+    }).toThrow('No such shape: this-shape-does-not-exist.');
+  });
+  it('should error on internal-only shape', function () {
+    expect(() => {
+      // this shape does exist, but it's only supposed to be for internal/backwards compatibility use
+      flow.parser.parse(`flowchart TB
+      A@{ shape: rect_left_inv_arrow }
+      `);
+    }).toThrow('No such shape: rect_left_inv_arrow. Shape names should be lowercase.');
+  });
   it('Diamond shapes should work as usual', function () {
     const res = flow.parser.parse(`flowchart TB
       A{This is a label}
diff --git a/packages/mermaid/src/diagrams/flowchart/types.ts b/packages/mermaid/src/diagrams/flowchart/types.ts
index 770ee24b46..b2c5cf6202 100644
--- a/packages/mermaid/src/diagrams/flowchart/types.ts
+++ b/packages/mermaid/src/diagrams/flowchart/types.ts
@@ -1,3 +1,28 @@
+import type { ShapeID } from '../../rendering-util/rendering-elements/shapes.js';
+
+/**
+ * Valid `type` args to `yy.addVertex` taken from
+ * `packages/mermaid/src/diagrams/flowchart/parser/flow.jison`
+ */
+export type FlowVertexTypeParam =
+  | undefined
+  | 'square'
+  | 'doublecircle'
+  | 'circle'
+  | 'ellipse'
+  | 'stadium'
+  | 'subroutine'
+  | 'rect'
+  | 'cylinder'
+  | 'round'
+  | 'diamond'
+  | 'hexagon'
+  | 'odd'
+  | 'trapezoid'
+  | 'inv_trapezoid'
+  | 'lean_right'
+  | 'lean_left';
+
 export interface FlowVertex {
   classes: string[];
   dir?: string;
@@ -10,7 +35,7 @@ export interface FlowVertex {
   props?: any;
   styles: string[];
   text?: string;
-  type?: string;
+  type?: ShapeID | FlowVertexTypeParam;
   icon?: string;
   form?: string;
   pos?: 't' | 'b';
diff --git a/packages/mermaid/src/rendering-util/rendering-elements/nodes.ts b/packages/mermaid/src/rendering-util/rendering-elements/nodes.ts
index 071776df20..e2eea5e198 100644
--- a/packages/mermaid/src/rendering-util/rendering-elements/nodes.ts
+++ b/packages/mermaid/src/rendering-util/rendering-elements/nodes.ts
@@ -23,7 +23,7 @@ export async function insertNode(elem: SVGGroup, node: Node, renderOptions: Shap
     }
   }
 
-  const shapeHandler = shapes[(node.shape ?? 'undefined') as keyof typeof shapes];
+  const shapeHandler = node.shape ? shapes[node.shape] : undefined;
 
   if (!shapeHandler) {
     throw new Error(`No such shape: ${node.shape}. Please check your syntax.`);
diff --git a/packages/mermaid/src/rendering-util/rendering-elements/shapes.ts b/packages/mermaid/src/rendering-util/rendering-elements/shapes.ts
index 40f9c7199c..4f6459d852 100644
--- a/packages/mermaid/src/rendering-util/rendering-elements/shapes.ts
+++ b/packages/mermaid/src/rendering-util/rendering-elements/shapes.ts
@@ -500,4 +500,8 @@ const generateShapeMap = () => {
 
 export const shapes = generateShapeMap();
 
+export function isValidShape(shape: string): shape is ShapeID {
+  return shape in shapes;
+}
+
 export type ShapeID = keyof typeof shapes;
diff --git a/packages/mermaid/src/rendering-util/types.ts b/packages/mermaid/src/rendering-util/types.ts
index 074e2c4a2c..e49218f711 100644
--- a/packages/mermaid/src/rendering-util/types.ts
+++ b/packages/mermaid/src/rendering-util/types.ts
@@ -1,5 +1,6 @@
 export type MarkdownWordType = 'normal' | 'strong' | 'em';
 import type { MermaidConfig } from '../config.type.js';
+import type { ShapeID } from './rendering-elements/shapes.js';
 export interface MarkdownWord {
   content: string;
   type: MarkdownWordType;
@@ -37,7 +38,7 @@ export interface Node {
   linkTarget?: string;
   tooltip?: string;
   padding?: number; //REMOVE?, use from LayoutData.config - Keep, this could be shape specific
-  shape?: string;
+  shape?: ShapeID;
   isGroup: boolean;
   width?: number;
   height?: number;