Error '438' in Excel 2016 with combo box

Our company is moving from Office 2013 to 2016 and as part of this process we are testing our existing Excel based programs to ensure they all work in the new Office. I've found with one particular program there is a combo box on one sheet that breaks the code in Workbook Open when trying to clear the combobox. The error '438' is received.
We've checked everything we can think of, the sheet is referenced before the combo box "Sheets("Sheet1").ComboBox1.clear" , the correct references are enabled, the sheet is unprotected. All the other comboboxes in the workbook are fine, just this one falls over.

Any ideas? Unfortunately I can't post the workbook here due to sensitive information.

I would have tried this but a colleague has decided to remove the combobox and recreate in the new version of Office and it seems to have fixed it. Not sure why it didn't work in the first place :/