diff --git a/packages/mermaid/src/diagrams/block/blockDB.ts b/packages/mermaid/src/diagrams/block/blockDB.ts
index 6b22d3ca0..2902af417 100644
--- a/packages/mermaid/src/diagrams/block/blockDB.ts
+++ b/packages/mermaid/src/diagrams/block/blockDB.ts
@@ -83,7 +83,7 @@ export const setCssClass = function (itemIds: string, cssClassName: string) {
   });
 };
 
-const populateBlockDatabase = (_blockList: Block[], parent: Block): void => {
+const populateBlockDatabase = (_blockList: (Block[] | Block[][]), parent: Block): void => {
   const blockList = _blockList.flat();
   const children = [];
   for (const block of blockList) {