GraphQL mutation from Power Query

Working in Excel Power Query. I can query data just fine in Power Query from a GraphQL API. But now I'm trying send to an API with a GraphQL mutation. Something causes it to duplicate the record on the endpoint:

let
    Source = Web.Contents(
	"https://api.######.com/v2",
	[
		Headers=[
			#"Method"="POST",
			#"Content-Type"="application/json",
			#"Authorization"=APIKey
		],
		Content=Text.ToBinary("{""query"": ""mutation { create_item (board_id:#####, group_id:#####, item_name:wazoo) { id } }""}")
	]
    ),
    #"JSON" = Json.Document(Source)
in
   #"JSON"

The #JSON line is from a version where I'm actually fetching the JSON. I don't really need it since I'm not parsing here, only sending/updating. So I tried to just returned Source in the let statement. This seems to fire the mutation query only once (yay!) but it throws an error in Excel, because Excel wants something to do.

The M:

let
    Source = Web.Contents(
	"https://api.######.com/v2",
	[
		Headers=[
			#"Method"="POST",
			#"Content-Type"="application/json",
			#"Authorization"=APIKey
	],
     Content=Text.ToBinary("{""query"": ""mutation { create_item (board_id:#####, group_id:new_group51930, item_name:wazoo) { id } }""}")
	]
    )
 in
       Source

I'm trying to learn how to work with APIs within Power Query. Info out there is sparse, appreciate any help and resources!