Change Table in a list based on the presence of a specific column

[SoftwareTester]

I retrieved values for several series of options from a few webpages with
tblOptionTables = Table.AddColumn(tblPages, "OptionTables" , each Web.Page(Web.Contents([Pages])){0}[Data]),
lis = tblOptionTables[OptionTables]              // a List of Tables  first column being "Calls" if valid otherwise "Column1"

Within tblOptionTables I want to replace each reference to a table (all being different tables of course) by null if it's not a valid table.
A table is valid if it's first column is "Calls".

At best it might be that while generating tblOptionTables null will be added instead of the table {0}[Data] if the table is not valid. 

how can I acchieve what i want?

[wdx223_Daniel]

=List.Transform(lis,each if Table.ColumnNames(_){0}="Calls" then _ else null)