Applying a single leaflet search control to search multiple layers?

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I am attempting to add a single search button to look through geoJSON layers. They will look through the field "name". Unfortunately, my method of creating a layer group is not working. What am I missing? Or how do I group together geoJSON files to make this work?

var searchControl = new L.Control.Search({ layer: [syriaLayer, countryBoundary], zoom: 7.5, propertyName: 'Sheet_Numb', circleLocation: false });

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