Fix runtime error: index out of range [0] with length 0 in Go/Echo

This error occurs when Go code accesses an index on an empty slice, such as items[0] when the slice has length 0. In Echo handlers, this commonly happens when a database query returns no results and the code accesses the first element without checking. Fix it by checking the slice length before accessing elements by index.

Reading the Stack Trace

Here's what each line means:

• main.GetLatestOrder(0xc000182000): The GetLatestOrder handler at line 22 of order.go accessed index 0 on an empty slice.
• runtime error: index out of range [0] with length 0: The Go runtime detected an out-of-bounds access: trying to access the first element of a slice that has zero elements.
• http: panic serving 127.0.0.1:52412: The panic caused by the out-of-range access was caught by the HTTP server, which logged it and returned a 500 error.

Common Causes

func GetLatestOrder(c echo.Context) error {
	userID := c.Param("user_id")
	orders, err := db.GetOrdersByUserID(userID)
	if err != nil {
		return c.JSON(500, map[string]string{"error": "database error"})
	}
	latest := orders[0] // panics if orders is empty
	return c.JSON(200, latest)
}

func ParseHeader(c echo.Context) error {
	auth := c.Request().Header.Get("Authorization")
	parts := strings.Split(auth, " ")
	token := parts[1] // panics if auth is empty or has no space
	return c.JSON(200, map[string]string{"token": token})
}

The Fix

Check len(orders) before accessing orders[0]. If the slice is empty, return a 404 response instead of panicking. This simple bounds check prevents the runtime panic entirely.

func GetLatestOrder(c echo.Context) error {
	userID := c.Param("user_id")
	orders, err := db.GetOrdersByUserID(userID)
	if err != nil {
		return c.JSON(500, map[string]string{"error": "database error"})
	}
	latest := orders[0]
	return c.JSON(200, latest)
}

func GetLatestOrder(c echo.Context) error {
	userID := c.Param("user_id")
	orders, err := db.GetOrdersByUserID(userID)
	if err != nil {
		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "database error"})
	}
	if len(orders) == 0 {
		return c.JSON(http.StatusNotFound, map[string]string{"error": "no orders found"})
	}
	latest := orders[0]
	return c.JSON(http.StatusOK, latest)
}

Testing the Fix

package handlers

import (
	"encoding/json"
	"net/http"
	"net/http/httptest"
	"testing"

	"github.com/labstack/echo/v4"
)

func TestGetLatestOrder_WithOrders(t *testing.T) {
	e := echo.New()
	req := httptest.NewRequest(http.MethodGet, "/", nil)
	rec := httptest.NewRecorder()
	c := e.NewContext(req, rec)
	c.SetPath("/api/users/:user_id/orders/latest")
	c.SetParamNames("user_id")
	c.SetParamValues("1")

	err := GetLatestOrder(c)
	if err != nil {
		t.Fatalf("handler returned error: %v", err)
	}
	if rec.Code != http.StatusOK {
		t.Errorf("expected status 200, got %d", rec.Code)
	}
}

func TestGetLatestOrder_NoOrders(t *testing.T) {
	e := echo.New()
	req := httptest.NewRequest(http.MethodGet, "/", nil)
	rec := httptest.NewRecorder()
	c := e.NewContext(req, rec)
	c.SetPath("/api/users/:user_id/orders/latest")
	c.SetParamNames("user_id")
	c.SetParamValues("99999")

	err := GetLatestOrder(c)
	if err != nil {
		t.Fatalf("handler returned error: %v", err)
	}
	if rec.Code != http.StatusNotFound {
		t.Errorf("expected status 404, got %d", rec.Code)
	}

	var body map[string]string
	json.Unmarshal(rec.Body.Bytes(), &body)
	if body["error"] != "no orders found" {
		t.Errorf("expected 'no orders found', got '%s'", body["error"])
	}
}

func TestGetLatestOrder_DoesNotPanic(t *testing.T) {
	defer func() {
		if r := recover(); r != nil {
			t.Errorf("handler panicked: %v", r)
		}
	}()

	e := echo.New()
	req := httptest.NewRequest(http.MethodGet, "/", nil)
	rec := httptest.NewRecorder()
	c := e.NewContext(req, rec)
	c.SetPath("/api/users/:user_id/orders/latest")
	c.SetParamNames("user_id")
	c.SetParamValues("nonexistent")

	GetLatestOrder(c)
}

Run your tests:

go test ./...

Pushing Through CI/CD

git checkout -b fix/index-out-of-range,git add handlers/order.go handlers/order_test.go,git commit -m "fix: check slice length before accessing first element",git push origin fix/index-out-of-range

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-go@v5
        with:
          go-version: '1.22'
      - run: go mod download
      - run: go vet ./...
      - run: go test -race -coverprofile=coverage.out ./...
      - run: go build ./...

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

go get github.com/bugstack/sdk

import "github.com/bugstack/sdk"

func init() {
  bugstack.Init(os.Getenv("BUGSTACK_API_KEY"))
}

Deploying the Fix (Manual Path)

1. Run go test ./... locally to confirm the bounds check works.
2. Run go vet ./... to catch other potential issues.
3. Open a pull request with the length check fix.
4. Have a teammate review and approve the PR.
5. Merge to main and verify the endpoint returns proper 404 responses in staging.

Frequently Asked Questions