Assignment to Entry in Nil Map

Why This Happens

In Go, declaring a map variable with var m map[string]int gives you a nil map. Reading from a nil map returns the zero value, but writing to it causes a panic. You must initialize it with make() or a map literal before use.

The Problem

func main() {
    var m map[string]int
    m["count"] = 1 // panic: assignment to entry in nil map
}

The Fix

func main() {
    m := make(map[string]int)
    m["count"] = 1 // works fine
}

Step-by-Step Fix

1. 1

   Identify the nil map

   Find the map variable in the panic stack trace and check how it was declared.

2. 2

   Trace the declaration

   Look for var declarations or struct fields of map type that are never initialized.

3. 3

   Initialize with make

   Use make(map[K]V) or a map literal to initialize the map before writing to it.