Formula.Firewall: Query references other queries, so it may not directly access a data source.

OK, I've read a little bit about this error but wasn't able to find anything exactly equivalent to what I am running into. Basically, all I was trying to do was add a global parameter so that I could easily change the source of my Folder ingest path. 

This query works great:

let
Source = Folder.Files("c:\powerbi\dev\Ingest\Monthly\FN"),
#"Invoke Custom Function1" = Table.AddColumn(Source, "Transform File from Test (2)", each #"FN Transform File"([Content])),
#"Renamed Columns1" = Table.RenameColumns(#"Invoke Custom Function1", {"Name", "Source.Name"}),
#"Removed Other Columns1" = Table.SelectColumns(#"Renamed Columns1", {"Source.Name", "Transform File from Test (2)"}),
#"Expanded Table Column1" = Table.ExpandTableColumn(#"Removed Other Columns1", "Transform File from Test (2)", Table.ColumnNames(#"FN Transform File"(#"FN Sample File"))),
...

The minute I do this (referencing a parameter in the Source line:

let
Source = Folder.Files(#"Root Path" & "\Ingest\Monthly\FN"),
#"Invoke Custom Function1" = Table.AddColumn(Source, "Transform File from Test (2)", each #"FN Transform File"([Content])),
#"Renamed Columns1" = Table.RenameColumns(#"Invoke Custom Function1", {"Name", "Source.Name"}),
#"Removed Other Columns1" = Table.SelectColumns(#"Renamed Columns1", {"Source.Name", "Transform File from Test (2)"}),
#"Expanded Table Column1" = Table.ExpandTableColumn(#"Removed Other Columns1", "Transform File from Test (2)", Table.ColumnNames(#"FN Transform File"(#"FN Sample File"))),
...

I get the error on the "Expanded Table Column 1" line. Even after reading about this error in a couple blog posts, I do not see how I am running afoul of the formula police and how to fix it.

Any suggestions out there?