diff --git a/packages/mermaid/src/diagrams/class/classDb.ts b/packages/mermaid/src/diagrams/class/classDb.ts
index 6d8453dd0..9c8a642c6 100644
--- a/packages/mermaid/src/diagrams/class/classDb.ts
+++ b/packages/mermaid/src/diagrams/class/classDb.ts
@@ -88,7 +88,7 @@ export const addClass = function (_id: string) {
     label: name,
     text: `${name}${type ? `<${type}>` : ''}`,
     shape: 'classBox',
-    cssClasses: ['default'],
+    cssClasses: 'default',
     methods: [],
     members: [],
     annotations: [],
@@ -279,7 +279,7 @@ export const setCssClass = function (ids: string, className: string) {
     }
     const classNode = classes.get(id);
     if (classNode) {
-      classNode.cssClasses.push(className);
+      classNode.cssClasses += ' ' + className;
     }
   });
 };
diff --git a/packages/mermaid/src/diagrams/class/classTypes.ts b/packages/mermaid/src/diagrams/class/classTypes.ts
index df2c9c0d2..9d0d47569 100644
--- a/packages/mermaid/src/diagrams/class/classTypes.ts
+++ b/packages/mermaid/src/diagrams/class/classTypes.ts
@@ -7,7 +7,7 @@ export interface ClassNode {
   label: string;
   shape: string;
   text: string;
-  cssClasses: string[];
+  cssClasses: string;
   methods: ClassMember[];
   members: ClassMember[];
   annotations: string[];