diff --git a/packages/mermaid/src/diagrams/class/classDb.ts b/packages/mermaid/src/diagrams/class/classDb.ts
index 4b2231dc1..bf326970e 100644
--- a/packages/mermaid/src/diagrams/class/classDb.ts
+++ b/packages/mermaid/src/diagrams/class/classDb.ts
@@ -464,9 +464,9 @@ export const setCssStyle = function (id: string, styles: string[]) {
   }
   for (const s of styles) {
     if (s.includes(',')) {
-      thisClass.styles = thisClass.styles.concat(s.split(','));
+      thisClass.styles = [...thisClass.styles, ...s.split(',')];
     } else {
-      thisClass.styles.push(s);
+      thisClass.styles?.push(s);
     }
   }
 };
diff --git a/packages/mermaid/src/diagrams/class/classTypes.ts b/packages/mermaid/src/diagrams/class/classTypes.ts
index 3b6c3694e..85be3a4e8 100644
--- a/packages/mermaid/src/diagrams/class/classTypes.ts
+++ b/packages/mermaid/src/diagrams/class/classTypes.ts
@@ -10,12 +10,12 @@ export interface ClassNode {
   members: ClassMember[];
   annotations: string[];
   domId: string;
+  styles: string[];
   parent?: string;
   link?: string;
   linkTarget?: string;
   haveCallback?: boolean;
   tooltip?: string;
-  styles?: string[];
 }
 
 export type Visibility = '#' | '+' | '~' | '-' | '';