diff --git a/v2/delay/delay.go b/v2/delay/delay.go
index 0b0393d..f9f3fa9 100644
--- a/v2/delay/delay.go
+++ b/v2/delay/delay.go
@@ -182,11 +182,11 @@
 func MustRegister(key string, i interface{}) *Function {
 	f, err := registerFunction(key, i)
 	if err != nil {
-		return f
+		panic(err)
 	}
 
 	if old := funcs[f.key]; old != nil {
-		panic(fmt.Sprintf("multiple functions registered for %q", key))
+		panic(errors.New(fmt.Sprintf("multiple functions registered for %q", key)))
 	}
 	funcs[f.key] = f
 	return f
diff --git a/v2/delay/delay_test.go b/v2/delay/delay_test.go
index c20d9d8..7708bef 100644
--- a/v2/delay/delay_test.go
+++ b/v2/delay/delay_test.go
@@ -42,9 +42,6 @@
 }
 
 var (
-	invalidFunc     = Func("invalid", func() {})
-	invalidRegister = MustRegister("invalid", func() {})
-
 	regFRuns = 0
 	regFMsg  = ""
 	regF     = func(c context.Context, arg string) {
@@ -101,13 +98,13 @@
 			dupeWhich = 1
 		}
 	}
-	dupe1Func     = Func("dupe", dupe1F)
-	dupe2F        = func(c context.Context) {
+	dupe1Func = Func("dupe", dupe1F)
+	dupe2F    = func(c context.Context) {
 		if dupeWhich == 0 {
 			dupeWhich = 2
 		}
 	}
-	dupe2Func     = Func("dupe", dupe2F)
+	dupe2Func = Func("dupe", dupe2F)
 
 	requestFuncRuns    = 0
 	requestFuncHeaders *taskqueue.RequestHeaders
@@ -151,11 +148,10 @@
 
 func TestInvalidFunction(t *testing.T) {
 	c := newFakeContext()
+	invalidFunc := Func("invalid", func() {})
 
-	for _, testTarget := range []*Function{invalidFunc, invalidRegister} {
-		if got, want := testTarget.Call(c.ctx), fmt.Errorf("delay: func is invalid: %s", errFirstArg); got.Error() != want.Error() {
-			t.Errorf("Incorrect error: got %q, want %q", got, want)
-		}
+	if got, want := invalidFunc.Call(c.ctx), fmt.Errorf("delay: func is invalid: %s", errFirstArg); got.Error() != want.Error() {
+		t.Errorf("Incorrect error: got %q, want %q", got, want)
 	}
 }
 
@@ -425,13 +421,33 @@
 func TestMustRegisterDuplicateFunction(t *testing.T) {
 	MustRegister("dupe", dupe1F)
 	defer func() {
-		if r := recover(); r == nil {
+		err := recover()
+		if err == nil {
 			t.Error("MustRegister did not panic")
 		}
+		got := fmt.Sprintf("%s", err)
+		want := fmt.Sprintf("multiple functions registered for %q", "dupe")
+		if got != want {
+			t.Errorf("Incorrect error: got %q, want %q", got, want)
+		}
 	}()
 	MustRegister("dupe", dupe2F)
 }
 
+func TestInvalidFunction_MustRegister(t *testing.T) {
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Error("MustRegister did not panic")
+		}
+		if err != errFirstArg {
+			t.Errorf("Incorrect error: got %q, want %q", err, errFirstArg)
+		}
+	}()
+	MustRegister("invalid", func() {})
+}
+
+
 func TestGetRequestHeadersFromContext(t *testing.T) {
 	for _, testTarget := range []*Function{requestFunc, requestRegister} {
 		c := newFakeContext()