What to use instead of air in empty loot table entries?

I'm upgrading my datapack to 1.21 and I got an error in the log saying "Item must not be minecraft:air" (attached screenshot). What should I do now? Here is the loot table in question: { "type": "minecraft:block", "pools": [ { "rolls": 1, "bonus_rolls": 0, "entries": [ { "type": "minecraft:alternatives", "children": [ { "type": "minecraft:item", "name": "minecraft:air" } ], "conditions": [ { "condition": "minecraft:location_check", "predicate": { "dimension": "limbo:limbo_dim" } } ] }, { "type": "minecraft:alternatives", "children": [ { "type": "minecraft:loot_table", "value": "limbo:blocks/gravel_old" } ], "conditions": [ { "condition": "minecraft:location_check", "predicate": { "dimension": "minecraft:overworld" } } ] } ] } ], "random_sequence": "minecraft:blocks/gravel" }