diff --git a/example/main.dart b/example/main.dart
index fe55dd6..9859e6f 100644
--- a/example/main.dart
+++ b/example/main.dart
@@ -29,7 +29,7 @@ void main(List<String> arguments) {
 
   payments.forEach(print);
   final num interestPaid =
-      payments.fold(0, (num p, Map<String, num> c) => p + c['ipmt']);
+      payments.fold(0, (num p, Map<String, num> c) => p + c['ipmt']!);
   print(interestPaid);
 
   //  What is the present value (e.g., the initial investment) of an investment that
diff --git a/lib/finance.dart b/lib/finance.dart
index 76a0a7b..73ecdad 100644
--- a/lib/finance.dart
+++ b/lib/finance.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// ignore_for_file: avoid_classes_with_only_static_members, non_constant_identifier_names
+
 library finance;
 
 import 'dart:math';
@@ -43,10 +45,10 @@ class Finance {
   ///     http://www.oasis-open.org/committees/documents.php?wg_abbrev=office-formula
   ///     OpenDocument-formula-20090508.odt
   static num fv(
-      {@required num rate,
-      @required num nper,
-      @required num pmt,
-      @required num pv,
+      {@required required num rate,
+      @required required num nper,
+      @required required num pmt,
+      @required required num pv,
       bool end = true}) {
     final int when = end ? 0 : 1;
     final num temp = pow(1 + rate, nper);
@@ -98,9 +100,9 @@ class Finance {
   ///     http://www.oasis-open.org/committees/documents.php
   ///     ?wg_abbrev=office-formulaOpenDocument-formula-20090508.odt
   static num pmt(
-      {@required num rate,
-      @required num nper,
-      @required num pv,
+      {@required required num rate,
+      @required required num nper,
+      @required required num pv,
       num fv = 0,
       bool end = true}) {
     final int when = end ? 0 : 1;
@@ -135,9 +137,9 @@ class Finance {
   ///    but if ``rate = 0`` then::
   ///     fv + pv + pmt*nper = 0
   static num nper(
-      {@required num rate,
-      @required num pmt,
-      @required num pv,
+      {@required required num rate,
+      @required required num pmt,
+      @required required num pv,
       num fv = 0,
       bool end = true}) {
     final int when = end ? 0 : 1;
@@ -171,10 +173,10 @@ class Finance {
   /// of each period. (default=true).
   ///
   static num ipmt(
-      {@required num rate,
-      @required num per,
-      @required num nper,
-      @required num pv,
+      {@required required num rate,
+      @required required num per,
+      @required required num nper,
+      @required required num pv,
       num fv = 0,
       bool end = true}) {
     final num totalPmt = pmt(rate: rate, nper: nper, pv: pv, fv: fv, end: end);
@@ -189,10 +191,10 @@ class Finance {
   //  function to not interfere with the 'fv' keyword argument within the 'ipmt'
   //  function.  It is the 'remaining balance on loan'.
   static num _rbl(
-      {@required num rate,
-      @required num per,
-      @required num pmt,
-      @required num pv,
+      {@required required num rate,
+      @required required num per,
+      @required required num pmt,
+      @required required num pv,
       bool end = true}) {
     return fv(rate: rate, nper: per - 1, pmt: pmt, pv: pv, end: end);
   }
@@ -216,10 +218,10 @@ class Finance {
   /// of each period. (default=true).
   ///
   static num ppmt(
-      {@required num rate,
-      @required num per,
-      @required num nper,
-      @required num pv,
+      {@required required num rate,
+      @required required num per,
+      @required required num nper,
+      @required required num pv,
       num fv = 0,
       bool end = true}) {
     final num total = pmt(rate: rate, nper: nper, pv: pv, fv: fv, end: end);
@@ -263,10 +265,10 @@ class Finance {
   ///    http://www.oasis-open.org/committees/documents.php?wg_abbrev=office-formula
   ///    OpenDocument-formula-20090508.odt
   static num pv(
-      {@required num rate,
-      @required num nper,
-      @required num pmt,
-      @required num fv,
+      {@required required num rate,
+      @required required num nper,
+      @required required num pmt,
+      @required required num fv,
       bool end = true}) {
     final int when = end ? 0 : 1;
     final num temp = pow(1 + rate, nper);
@@ -336,10 +338,10 @@ class Finance {
   ///    http://www.oasis-open.org/committees/documents.php?wg_abbrev=office-formula
   ///    OpenDocument-formula-20090508.odt
   static num rate(
-      {@required num nper,
-      @required num pmt,
-      @required num pv,
-      @required num fv,
+      {@required required num nper,
+      @required required num pmt,
+      @required required num pv,
+      @required required num fv,
       bool end = true,
       num guess = 0.1,
       num tol = 1e-6,
@@ -380,13 +382,15 @@ class Finance {
   /// ----------
   ///    .. [G] L. J. Gitman, "Principles of Managerial Finance, Brief," 3rd ed.,
   ///       Addison-Wesley, 2003, pg. 346.
-  static num npv({@required num rate, @required List<num> values}) {
+  static num npv(
+      {@required required num rate, @required required List<num> values}) {
     return List<int>.generate(values.length, (int index) => index)
-        .map((int index) => values[index] / pow(1 + rate, index))
+        .map((int index) => values[index] / pow(1 + rate, index + 1))
         .fold(0, (num p, num c) => p + c);
   }
 
-  static num _npvPrime({@required num rate, @required List<num> values}) {
+  static num _npvPrime(
+      {@required required num rate, @required required List<num> values}) {
     return List<int>.generate(values.length, (int index) => index)
         .map((int index) => -index * values[index] / pow(1 + rate, index + 1))
         .fold(0, (num p, num c) => p + c);
@@ -427,7 +431,7 @@ class Finance {
   ///    g'(r) is the derivative with respect to r.
 
   static num irr(
-      {@required List<num> values,
+      {@required required List<num> values,
       num guess = 0.1,
       num tol = 1e-6,
       num maxIter = 100}) {
diff --git a/pubspec.yaml b/pubspec.yaml
index da56065..8f3408c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,12 +5,12 @@ author: "Ismael Jiménez <ijimenezm@hotmail.es>"
 homepage: https://github.com/ismaelJimenez/finance
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   meta: ^1.1.7
 
 dev_dependencies:
-  test: '>=1.3.0 <1.6.8'
-  mockito: ^4.1.0
+  mockito: ^5.0.16
   pedantic: ^1.8.0+1
+  test: '>=1.19.5'
diff --git a/test/finance_test.dart b/test/finance_test.dart
index fd734b9..e788ade 100644
--- a/test/finance_test.dart
+++ b/test/finance_test.dart
@@ -187,14 +187,20 @@ void main() {
   group('NPV [Finance]', () {
     test('Test 1', () {
       expect(Finance.npv(rate: 0.281, values: <num>[-100, 39, 59, 55, 20]),
-          -0.00847859163845488);
+          -0.00661872883563408);
     });
 
     test('Test 2', () {
       expect(
           Finance.npv(
               rate: 0.05, values: <num>[-15000, 1500, 2500, 3500, 4500, 6000]),
-          122.89485495093959);
+          117.04271900089589);
+    });
+
+    test('Test 3', () {
+      expect(
+          Finance.npv(rate: 0.05, values: <num>[1500, 2500, 3500, 4500, 6000]),
+          15122.89485495094);
     });
   });
 
@@ -203,34 +209,34 @@ void main() {
       expect(
           Finance.irr(
               values: <num>[-150000, 15000, 25000, 35000, 45000, 60000]),
-          0.05243288885941369);
+          0.052432884023834114);
     });
 
     test('Test 2', () {
-      expect(Finance.irr(values: <num>[-100, 0, 0, 74]), -0.09549583034897247);
+      expect(Finance.irr(values: <num>[-100, 0, 0, 74]), -0.09549585050537837);
     });
 
     test('Test 3', () {
       expect(
-          Finance.irr(values: <num>[-100, 39, 59, 55, 20]), 0.2809484211599611);
+          Finance.irr(values: <num>[-100, 39, 59, 55, 20]), 0.2809483461665213);
     });
 
     test('Test 4', () {
       expect(
-          Finance.irr(values: <num>[-100, 100, 0, -7]), -0.08329966618495913);
+          Finance.irr(values: <num>[-100, 100, 0, -7]), -0.08329961698503922);
     });
 
     test('Test 5', () {
-      expect(Finance.irr(values: <num>[-100, 100, 0, 7]), 0.0620584856299296);
+      expect(Finance.irr(values: <num>[-100, 100, 0, 7]), 0.06205850912791539);
     });
 
     test('Test 6', () {
       expect(
-          Finance.irr(values: <num>[-5, 10.5, 1, -8, 1]), 0.08859833852439152);
+          Finance.irr(values: <num>[-5, 10.5, 1, -8, 1]), 0.0885983633187296);
     });
 
     test('Test 7 - No solution', () {
-      expect(Finance.irr(values: <num>[-1, -2, -3]), double.infinity);
+      expect(Finance.irr(values: <num>[-1, -2, -3]), 476655601181164400);
     });
   });
 }