How to Fix Go Nil Map Assignment Panics

In this tutorial, you'll learn about How to Fix Go Nil Map Assignment Panics. We cover key concepts, practical examples, and best practices.

Go panics with assignment to entry in nil map when you write to a map variable that has been declared but not initialized with make or a composite literal. Reading from a nil map returns the zero value without panicking, but writing causes a runtime crash.

Quick Fix

Wrong

var scores map[string]int
scores["Alice"] = 95

panic: assignment to entry in nil map

The map is declared but not initialized. A nil map cannot accept writes.

Right

scores := make(map[string]int)
scores["Alice"] = 95
fmt.Println(scores["Alice"])

95

Fix with composite literal

scores := map[string]int{
    "Alice": 95,
    "Bob":   87,
}
scores["Charlie"] = 92
fmt.Println(scores)

map[Alice:95 Bob:87 Charlie:92]

Fix in struct initialization

type Team struct {
    Name   string
    Scores map[string]int
}

// Wrong:
team := Team{Name: "Red"}
team.Scores["Alice"] = 95
// panic: assignment to entry in nil map

// Right:
team := Team{
    Name:   "Red",
    Scores: make(map[string]int),
}
team.Scores["Alice"] = 95

Fix when returning from function

func newScores() map[string]int {
    return make(map[string]int)
}

func main() {
    s := newScores()
    s["Alice"] = 95
}

Prevention

• Always initialize maps with make() before writing to them.
• Use composite literals map[K]V{...} for maps with initial values.
• Check for nil before writing in library code: if m == nil { m = make(map[K]V) }.
• Initialize maps inside struct constructors.
• Run go vet to detect potential nil map assignments.

DodaTech Tools

Doda Browser's Go analysis tool detects nil map assignments at compile time. DodaZIP archives Go module source code for static analysis. Durga Antivirus Pro monitors for nil map panics in production logs.

Common Mistakes with nil map error

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ