OData error calling function when identical table works fine

Hi, I have an odata function which I've setup to return exactly the same data as a entity table (verified json is the same through fiddler). However, the function call gives a json error, whist the table call works. If i reduce the size of one of the string fields, then the json parses no problem when called from the function. There seems to be something strange with the call to the odata function truncating the json value.

This works:

let
Source = OData.Feed("https://MYIP/odata", null, [Implementation="2.0"]),
tableData= Source{[Name="MYTABLE",Signature="table"]}[Data],
#"Kept First Rows" = Table.FirstN(tableData,1)
in
#"Kept First Rows"

This does not:

let
Source = OData.Feed("https://MYIP/odata", null, [Implementation="2.0"]),
#"Top_function (NumberToReturn as number) as table" = Source{[Name="Top",Signature="function (NumberToReturn as number) as table"]},
Data = #"Top_function (NumberToReturn as number) as table"[Data],
#"Invoked FunctionData" = Data(1)
in
#"Invoked FunctionData"

In the web API they both generate exactly the same data - confirmed by fiddler on both requests.

The error is:

DataSource.Error: OData: Invalid JSON. Unexpected end of input reached while processing a JSON string value.
Details:
DataSourceKind=OData