How to pass a chart variable to a function

I’m using VBA for Access 2010 to create charts. Since I’m using the same chart on both a form and a report, I thought I would create a function that builds the chart, then call this function from both the form and the report. But I’m not able to pass the chart to the function.
I have tried

Public Function initChart(chartResults As Chart2D)

but I get "Type mismatch" during the call to the function.

Any ideas?

Thanks,

jan