'str' object has no attribute 'items' when visualising Vega plot of network using Panel

[jandot]

When running pn.pane.Vega(spec) on a network spec (from the vega example showing a force-directed network of the Miserables characters), I get an error in line 21 of the pane.py source file. The error:

~/anaconda3/envs/i0u19a_jupyter/lib/python3.7/site-packages/panel/pane/vega.py in ds_as_cds(dataset)
     19     if len(dataset) == 0:
     20         return {}
---> 21     data = {k: [] for k, v in dataset[0].items()}
     22     for item in dataset:
     23         for k, v in item.items():

AttributeError: 'str' object has no attribute 'items'

See the image for a screenshot of the complete error message. The command used was pn.pane.Vega(spec), the error what is shown above, and the output was Vega(dict).