Expression.Error: 3 arguments were passed to a function which expects between 1 and 2. - help please

I'm getting this error which is driving me nuts that I'm hoping somebody can spot just what I'm doing wrong.  I'm sure it must be something to do with the dates I'm using, but what?  Everything I've tried keeps coming back to the same problem.  

Here's the function that is being called: 

let
Source2 = (MyStartDate as date, MyEndDate as date, MyTimeUnit as text, MyAPIKey as text) as table =>
let
Source = Json.Document(Web.Contents("https://monitoringapi.solaredge.com",
[
RelativePath="site/123456/energy"
],
[
Query= "startDate=" & Date.ToText(MyStartDate, "yyyy-mm-dd")
& "&endDate=" & Date.ToText(MyEndDate, "yyyy-mm-dd")
& "&timeUnit=" & MyTimeUnit
& "&api_key=" & MyAPIKey
]
)
),
#"Converted to Table" = Record.ToTable(Source),
#"Expanded Value" = Table.ExpandRecordColumn(#"Converted to Table", "Value", {"timeUnit", "unit", "measuredBy", "values"}, {"timeUnit", "unit", "measuredBy", "values"}),
#"Expanded values" = Table.ExpandListColumn(#"Expanded Value", "values"),
#"Expanded values1" = Table.ExpandRecordColumn(#"Expanded values", "values", {"date", "value"}, {"date", "value"})
in
#"Expanded values1"
in
Source2

And here is the call: 

let
Source = URL,
#"Invoked Custom Function" = Table.AddColumn(Source, "Usage", each Usage([StartDate], [EndDate], [TimeUnit], [api_key]))
in
#"Invoked Custom Function"

Timeunit is just some text ("quarter-of-an-hour") and the api_key just text.  StartDate & EndDate are dates.  

Can anybody spot anything that I might change here?  

Thanks in advance.