diff --git a/packages/mermaid/src/diagrams/quadrant-chart/parser/quadrant.jison.spec.ts b/packages/mermaid/src/diagrams/quadrant-chart/parser/quadrant.jison.spec.ts
index 6ec2ddafb..e59cc0fe2 100644
--- a/packages/mermaid/src/diagrams/quadrant-chart/parser/quadrant.jison.spec.ts
+++ b/packages/mermaid/src/diagrams/quadrant-chart/parser/quadrant.jison.spec.ts
@@ -2,6 +2,7 @@
 import { parser } from './quadrant.jison';
 import type { Mock } from 'vitest';
 import { vi } from 'vitest';
+import { addClass } from '../../flowchart/flowDb.js';
 
 const parserFnConstructor = (str: string) => {
   return () => {
@@ -20,6 +21,7 @@ const mockDB: Record<string, Mock<any, any>> = {
   setYAxisBottomText: vi.fn(),
   setDiagramTitle: vi.fn(),
   addPoint: vi.fn(),
+  addClass: vi.fn(),
 };
 
 function clearMocks() {
@@ -423,4 +425,13 @@ describe('Testing quadrantChart jison file', () => {
       ['stroke-width: 10px']
     );
   });
+
+  it('should be able to handle constructor as a className', () => {
+    const str = `quadrantChart
+    classDef constructor fill:#ff0000
+    Microsoft:::constructor: [0.75, 0.75]
+    `;
+    expect(parserFnConstructor(str)).not.toThrow();
+    expect(mockDB.addClass).toHaveBeenCalledWith('constructor', ['fill:#ff0000']);
+  });
 });
diff --git a/packages/mermaid/src/diagrams/quadrant-chart/quadrantBuilder.ts b/packages/mermaid/src/diagrams/quadrant-chart/quadrantBuilder.ts
index 173b4c078..f1507a1b9 100644
--- a/packages/mermaid/src/diagrams/quadrant-chart/quadrantBuilder.ts
+++ b/packages/mermaid/src/diagrams/quadrant-chart/quadrantBuilder.ts
@@ -127,7 +127,7 @@ export class QuadrantBuilder {
   private config: QuadrantBuilderConfig;
   private themeConfig: QuadrantBuilderThemeConfig;
   private data: QuadrantBuilderData;
-  private classes: Record<string, StylesObject> = {};
+  private classes: Map<string, StylesObject> = new Map();
 
   constructor() {
     this.config = this.getDefaultConfig();
@@ -202,7 +202,7 @@ export class QuadrantBuilder {
     this.config = this.getDefaultConfig();
     this.themeConfig = this.getDefaultThemeConfig();
     this.data = this.getDefaultData();
-    this.classes = {};
+    this.classes = new Map();
     log.info('clear called');
   }
 
@@ -215,7 +215,7 @@ export class QuadrantBuilder {
   }
 
   addClass(className: string, styles: StylesObject) {
-    this.classes[className] = styles;
+    this.classes.set(className, styles);
   }
 
   setConfig(config: Partial<QuadrantBuilderConfig>) {
@@ -486,7 +486,7 @@ export class QuadrantBuilder {
       .range([quadrantHeight + quadrantTop, quadrantTop]);
 
     const points: QuadrantPointType[] = this.data.points.map((point) => {
-      const classStyles = this.classes[point.className as keyof typeof this.classes];
+      const classStyles = this.classes.get(point.className!);
       if (classStyles) {
         point = { ...classStyles, ...point };
       }