How to test value of headers User-Agent

[gonta1026]

I am not very familiar with golang.
Below is a simple function I wrote that returns the User-Agent value.

func getUserAgent(ctx context.Context) string {
	userAgent := graphql.GetOperationContext(ctx).Headers.Get("User-Agent")
	return userAgent
}

Having verified that I could get the User-Agent value, I wrote the getUserAgent test code.

type OperationContext struct {
	Headers http.Header
}
type operationCtx struct{}

func Test_getUserAgent(t *testing.T) {
	t.Run("getUserAgent", func(t *testing.T) {
		headers := map[string][]string{
			"User-Agent": {"User-Agent sample"},
		}
		ctx := context.WithValue(context.Background(), operationCtx{}, &OperationContext{
			Headers: headers,
		})
		userAgent := getUserAgent(ctx) // panic: missing operation context [recovered]
		assert.Equal(t, userAgent, "User-Agent sample")
	})
}

But when I run the test, I get the following error

--- FAIL: Test_getUserAgent (0.00s)
    --- FAIL: Test_getUserAgent/getUserAgent (0.00s)
panic: missing operation context [recovered]
	panic: missing operation context

What kind of test code should I write to solve this problem?