i got panic: runtime error: invalid memory address or nil pointer deference on for-loop condition fast!=nil && fast.Next!=nil when test with head=nil.

but it goes well if call middleNode from main or place this condition in if-else.

type Node struct{
    Value int
    Next *Node
}

func middleNode(head *Node) *Node {
    sentinel := &Node{Next:head}
    slow, fast := sentinel, sentinel
    for fast!=nil && fast.Next!=nil {
        slow = slow.Next
        fast = fast.Next
        if fast == nil{
            return slow
        }
        fast = fast.Next
        if fast == nil{
            return slow
        }
    }
    return slow
}

this’s my code