bokeh show series name when hovering over line on chart

I am plotting a line chart. I use the HoverTool so that when a user hovers over a line they can see the date & the value, this bit works. However it doesn't show them the series name, I have tried using the special $name but it just shows three question marks. What am I doing wrong?

source = ColumnDataSource(data=dict(
x=[dt.datetime(2023, 1, 1), dt.datetime(2023, 1, 2), dt.datetime(2023, 1, 3), dt.datetime(2023, 1, 4), dt.datetime(2023, 1, 5)],
y1=[2, 5, 8, 2, 7],
y2=[3, 2, 1, 4, 7],
y3=[1, 4, 5, 6, 3]
))

source = ColumnDataSource(data_dict)

tools = "pan,box_zoom,zoom_in,zoom_out,redo,undo,reset,crosshair"
p = figure(title = "Blah", 
           x_axis_label='date', 
           width=1600, 
           height=850,
           x_axis_type='datetime',
           tools=tools,
           toolbar_location='above')

for c in data_dict.keys():
    if c != 'x':
        p.line(x='x', y=c, source=source, legend_label=c)

p.legend.location = "top_left"
p.legend.click_policy = "hide"

formatters_tooltips = {'$x': 'datetime'}

p.add_tools(HoverTool(
    tooltips=[('Date', '$x{%Y-%m-%d}'), ('Name', '$name'), ('Value', '$y')],
    formatters=formatters_tooltips))

output_file("C:/some_path/some_file.html")
   
show(p)

I'm deducing you put the 'Name' placeholder in your tooltips, gave your plot a title, but nowhere is $name defined. Perhaps adding a 'name=' parameter to your p.line would name your series.