attempt to compare two userdata values?

The goal is that when a player presses e, they will move to the nearest grip point, and "climb" up a wall.

When the player presses their e key, this error occurs Players.Player1.Backpack.LocalScript:9: attempt to compare two userdata values

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

local function FindNearestGrip()
    local grips = game.Workspace:FindFirstChild("Grips", true):GetChildren()
    table.sort(grips, function(a, b)
    local dstA = (a.Position - player.Character.HumanoidRootPart.Position).magnitude
    local dstB = (b.Position - player.Character.HumanoidRootPart.Position).magnitude
return a < b
    end)
    return grips[1]
end

mouse.KeyDown:connect(function(key)
    if key == 'e' then 
        FindNearestGrip()
    end 
end)