Colspan issue with subplots - works with spanning across two columns, but not three


I am wanting to create a subplot with 4 rows and 3 columns. Each column for the first three rows is to contain an individual plot, whereas the fourth row will have one plot spanning three columns. This is the code I have so far, for which I intend to use the fig.add_trace command to add the individual plots:

fig = make_subplots(rows=4, cols=3,specs=[[{}, {}, {}],[{}, {}, {}],[{}, {}, {}],[{‘colspan’: 3}, None]])

However with this I get the following error: “The ‘specs’ argument to make_subplots must be a 2D list of dictionaries with dimensions (4 x 3). Received value of type <class ‘list’>: [[{}, {}, {}], [{}, {}, {}], [{}, {}, {}], [{‘colspan’: 3}, None]]”

I am unsure why this is - as far as I understand the specs is in the 4x3 arrangement. Interestingly, if I set the plot in the fourth row to span across only 2 rows (see below), the code works.

fig = make_subplots(rows=4, cols=3,specs=[[{}, {}, {}],[{}, {}, {}],[{}, {}, {}],[{‘colspan’: 2}, {}, None]])

Does anyone have an idea why this is not working for me?