## 🐛 Bug Report <!-- A clear and concise description of what the bug is. --> Lib version: 4.0.1 ## Steps To Reproduce <!-- The exact steps required to reproduce the issue, ideally with a code example --> Get a error when [clone worksheet](https://github.com/exceljs/exceljs/issues/591#issuecomment-455425086) with using typescript. ```typescript const tempSheet = tempWorkbook.getWorksheet('template') const clonedSheet = parentWorkbook.addWorksheet('sheet') clonedSheet.model = Object.assign(tempSheet.model, { mergeCells: tempSheet.model.merges }) clonedSheet.name = `a-unique-name` clonedSheet.eachRow(row => { row.eachCell(cell => { if (cell.master !== cell) { cell.style = cell.master.style } }) }) ``` ## The expected behaviour:  I must use `@ts-ignore` to disable the error. ```typescript clonedSheet.model = Object.assign(tempSheet.model, { /* @ts-ignore */ mergeCells: tempSheet.model.merges }) ``` I think that isn't consistent for using typescript. Is't possible to add `merges` property to `WorksheetModel` interface?