diff --git a/CliFx.Tests/ArgumentBindingSpecs.cs b/CliFx.Tests/ArgumentBindingSpecs.cs
index eb8b462..1d6873f 100644
--- a/CliFx.Tests/ArgumentBindingSpecs.cs
+++ b/CliFx.Tests/ArgumentBindingSpecs.cs
@@ -933,7 +933,7 @@ namespace CliFx.Tests
         }
 
         [Fact]
-        public void Property_annotated_as_a_required_option_must_always_be_bound_to_some_value()
+        public void Property_annotated_as_a_required_option_must_always_be_set()
         {
             // Arrange
             var schema = ApplicationSchema.Resolve(new[] {typeof(RequiredOptionCommand)});
@@ -946,6 +946,20 @@ namespace CliFx.Tests
             Assert.Throws<CliFxException>(() => schema.InitializeEntryPoint(input));
         }
 
+        [Fact]
+        public void Property_annotated_as_a_required_option_must_always_be_bound_to_some_value()
+        {
+            // Arrange
+            var schema = ApplicationSchema.Resolve(new[] {typeof(RequiredOptionCommand)});
+
+            var input = new CommandLineInputBuilder()
+                .AddOption(nameof(RequiredOptionCommand.OptionB))
+                .Build();
+
+            // Act & assert
+            Assert.Throws<CliFxException>(() => schema.InitializeEntryPoint(input));
+        }
+
         [Fact]
         public void Property_annotated_as_parameter_is_bound_directly_from_argument_value_according_to_the_order()
         {
diff --git a/CliFx/Domain/CommandSchema.cs b/CliFx/Domain/CommandSchema.cs
index cdc201b..682db9c 100644
--- a/CliFx/Domain/CommandSchema.cs
+++ b/CliFx/Domain/CommandSchema.cs
@@ -97,15 +97,15 @@ namespace CliFx.Domain
             var unsetRequiredOptions = Options.Where(o => o.IsRequired).ToList();
 
             // Environment variables
-            foreach (var environmentVariable in environmentVariables)
+            foreach (var (name, value) in environmentVariables)
             {
-                var option = Options.FirstOrDefault(o => o.MatchesEnvironmentVariableName(environmentVariable.Key));
+                var option = Options.FirstOrDefault(o => o.MatchesEnvironmentVariableName(name));
 
                 if (option != null)
                 {
                     var values = option.IsScalar
-                        ? new[] {environmentVariable.Value}
-                        : environmentVariable.Value.Split(Path.PathSeparator);
+                        ? new[] {value}
+                        : value.Split(Path.PathSeparator);
 
                     option.Inject(command, values);
                     unsetRequiredOptions.Remove(option);
@@ -122,12 +122,14 @@ namespace CliFx.Domain
 
                 if (inputs.Any())
                 {
-                    option.Inject(command, inputs.SelectMany(i => i.Values).ToArray());
+                    var inputValues = inputs.SelectMany(i => i.Values).ToArray();
+                    option.Inject(command, inputValues);
 
                     foreach (var input in inputs)
                         remainingOptionInputs.Remove(input);
 
-                    unsetRequiredOptions.Remove(option);
+                    if (inputValues.Any())
+                        unsetRequiredOptions.Remove(option);
                 }
             }
 
diff --git a/CliFx/Internal/Polyfills.cs b/CliFx/Internal/Polyfills.cs
index 126c272..9070038 100644
--- a/CliFx/Internal/Polyfills.cs
+++ b/CliFx/Internal/Polyfills.cs
@@ -1,18 +1,20 @@
 // ReSharper disable CheckNamespace
 
-#if NET45 || NETSTANDARD2_0
-using System.Collections.Generic;
-using System.Text;
+// Polyfills to bridge the missing APIs in older versions of the framework/standard.
 
-namespace CliFx.Internal
+#if NETSTANDARD2_0 || NET45
+namespace System.Collections.Generic
 {
-    internal static class Polyfills
+    internal static class Extensions
     {
-        public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> self, TKey key) =>
-            self.TryGetValue(key, out var value) ? value : default;
+        public static void Deconstruct<TKey, TValue>(this KeyValuePair<TKey, TValue> pair, out TKey key, out TValue value)
+        {
+            key = pair.Key;
+            value = pair.Value;
+        }
 
-        public static StringBuilder AppendJoin<T>(this StringBuilder self, string separator, IEnumerable<T> items) =>
-            self.Append(string.Join(separator, items));
+        public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dic, TKey key) =>
+            dic.TryGetValue(key, out var result) ? result! : default!;
     }
 }
 #endif
\ No newline at end of file