diff --git a/handler/handler.go b/handler/handler.go
index cb14e31..456ca82 100644
--- a/handler/handler.go
+++ b/handler/handler.go
@@ -15,6 +15,9 @@ type Handler struct {
 }
 
 func (this *Handler) Handle(args []string) error {
+	if this.calculator == nil {
+		return fmt.Errorf("%w: calculator is required", ErrNilCalculator)
+	}
 	if len(args) != 2 {
 		return fmt.Errorf("%w: two args req (you provided %d", ErrTooFewArgs, len(args))
 	}
@@ -23,6 +26,7 @@ func (this *Handler) Handle(args []string) error {
 	if err != nil {
 		return fmt.Errorf("%w: first arg (%s) %w", ErrMalformedArgs, a, err)
 	}
+
 	b, err := strconv.Atoi(args[1])
 	if err != nil {
 		return fmt.Errorf("%w: first arg (%s) %w", ErrMalformedArgs, b, err)
@@ -37,7 +41,7 @@ func (this *Handler) Handle(args []string) error {
 	return nil
 }
 
-func NewHandler(output io.Writer, calculator calc.Addition) *Handler {
+func NewHandler(output io.Writer, calculator calc.Calculator) *Handler {
 	return &Handler{
 		calculator: calculator,
 		output:     output,
@@ -48,4 +52,5 @@ var (
 	ErrTooFewArgs    = errors.New("\"Usage: <a> <b>\"")
 	ErrMalformedArgs = errors.New("invalid argument")
 	ErrOutputWriter  = errors.New("output write failed")
+	ErrNilCalculator = errors.New("nil calculator")
 )
diff --git a/main.go b/main.go
index c7fc903..0e0455f 100644
--- a/main.go
+++ b/main.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"flag"
 	"log"
 	"os"
 
@@ -9,8 +10,18 @@ import (
 )
 
 func main() {
-	err := handler.NewHandler(os.Stdout, calc.Addition{}).Handle(os.Args[1:])
+	var op string
+	flag.StringVar(&op, "op", "+", "One of + - * /")
+	flag.Parse()
+	err := handler.NewHandler(os.Stdout, calculators[op]).Handle(flag.Args())
 	if err != nil {
 		log.Fatal(err)
 	}
 }
+
+var calculators = map[string]calc.Calculator{
+	"+": calc.Addition{},
+	"-": calc.Subtraction{},
+	"*": calc.Multiplication{},
+	"/": calc.Division{},
+}
diff --git a/main_test.go b/main_test.go
index 95c03a0..b6fbde2 100644
--- a/main_test.go
+++ b/main_test.go
@@ -6,8 +6,8 @@ import (
 	"reflect"
 	"testing"
 
-	calc "calc-lib/calc"
-	handler "calc-lib/handler"
+	"github.com/JacobMcKenzieSmarty/calc-lib/calc"
+	"github.com/JacobMcKenzieSmarty/calc-lib/handler"
 )
 
 func assertEqual(t *testing.T, expected, actual any) {
@@ -51,6 +51,12 @@ func TestOutputWriterError(t *testing.T) {
 	assertError(t, taco, err)
 }
 
+func TestNilCalculator(t *testing.T) {
+	myHandler := handler.NewHandler(&bytes.Buffer{}, nil)
+	err := myHandler.Handle([]string{"1", "2"})
+	assertError(t, handler.ErrNilCalculator, err)
+}
+
 func TestHappyPath(t *testing.T) {
 	myBuffer := new(bytes.Buffer)
 	myHandler := handler.NewHandler(myBuffer, calc.Addition{})