diff --git a/Behavioral/DesignPatterns.Visitor.UnitTests/SizeVisitorTests.cs b/Behavioral/DesignPatterns.Visitor.UnitTests/SizeVisitorTests.cs
index fe65d52..ea8a26b 100644
--- a/Behavioral/DesignPatterns.Visitor.UnitTests/SizeVisitorTests.cs
+++ b/Behavioral/DesignPatterns.Visitor.UnitTests/SizeVisitorTests.cs
@@ -7,6 +7,14 @@ namespace DesignPatterns.Visitor.UnitTests;
 
 public class SizeVisitorTests
 {
+    [Fact]
+    public void GivenSize_WhenCallVisit_ThenInvalidOperationExceptionThrown()
+    {
+        var sizeVisitor = new SizeVisitor(Enumerable.Empty<ISizeConverter<ISize>>());
+        var action = () => sizeVisitor.Visit(default);
+        action.ShouldThrow<InvalidOperationException>();
+    }
+
     [Theory]
     [MemberData(nameof(TestCases))]
     public void GivenSize_WhenCallVisit_ThenResultAsExpected(ISize size, ISize expectedResult)
@@ -16,14 +24,14 @@ public void GivenSize_WhenCallVisit_ThenResultAsExpected(ISize size, ISize expec
         actualResult.ShouldBeEquivalentTo(expectedResult);
     }
 
-    private IVisitor<ISize, ISize> GetSut() => new SizeVisitor(
-    [
-        new KiloBytesToBytesConverter(),
-        new MegaBytesToBytesConverter(),
-        new GigaBytesToBytesConverter(),
-        new TeraBytesToBytesConverter(),
-        new PetaBytesToBytesConverter()
-    ]);
+    private IVisitor<ISize, ISize> GetSut() =>
+        new SizeVisitor([
+            new KiloBytesToBytesConverter(),
+            new MegaBytesToBytesConverter(),
+            new GigaBytesToBytesConverter(),
+            new TeraBytesToBytesConverter(),
+            new PetaBytesToBytesConverter()
+        ]);
 
     public static IEnumerable<object[]> TestCases()
     {
diff --git a/Behavioral/DesignPatterns.Visitor.UnitTests/StringVisitorTests.cs b/Behavioral/DesignPatterns.Visitor.UnitTests/StringVisitorTests.cs
index 2b8259b..9edcd62 100644
--- a/Behavioral/DesignPatterns.Visitor.UnitTests/StringVisitorTests.cs
+++ b/Behavioral/DesignPatterns.Visitor.UnitTests/StringVisitorTests.cs
@@ -24,4 +24,34 @@ public void GivenStringValue_WhenCallVisit_ThenResultAsExpected(string inputValu
         result.ShouldBeOfType(expectedType);
         result.Value.ShouldBe(expectedValue);
     }
-}
\ No newline at end of file
+
+    [Fact]
+    public void GivenNull_WhenCallVisit_ThenArgumentExceptionThrown()
+    {
+        var stringVisitor = new StringVisitor();
+        var action = () => stringVisitor.Visit(null!);
+        action.ShouldThrow<ArgumentException>();
+    }
+
+    [Theory]
+    [InlineData("1ZB")]
+    public void GivenInvalidValue_WhenCallVisit_ThenArgumentExceptionThrown(string value)
+    {
+        var stringVisitor = new StringVisitor();
+        var action = () => stringVisitor.Visit(value);
+        action
+            .ShouldThrow<ArgumentException>()
+            .Message.ShouldBe("Could not parse the provided value. (Parameter 'size')");
+    }
+
+    [Theory]
+    [InlineData("1EB")]
+    public void GivenUnknownSizeUnit_WhenCallVisit_ThenArgumentOutOfRangeExceptionThrown(string value)
+    {
+        var stringVisitor = new StringVisitor();
+        var action = () => stringVisitor.Visit(value);
+        action
+            .ShouldThrow<ArgumentOutOfRangeException>()
+            .Message.ShouldBe("Unknown size unit. (Parameter 'size')");
+    }
+}
diff --git a/Behavioral/DesignPatterns.Visitor/StringVisitor.cs b/Behavioral/DesignPatterns.Visitor/StringVisitor.cs
index e859bbc..8d39fa5 100644
--- a/Behavioral/DesignPatterns.Visitor/StringVisitor.cs
+++ b/Behavioral/DesignPatterns.Visitor/StringVisitor.cs
@@ -12,12 +12,12 @@ public ISize Visit(string size)
             throw new ArgumentException("Input size string cannot be null or empty.", nameof(size));
         }
 
-        var regexPattern = @"^(?<value>\d+(\.\d+)?)\s*(?<unit>[KMGTP]?B)$";
+        var regexPattern = @"^(?<value>\d+(\.\d+)?)\s*(?<unit>[KMGTPE]?B)$";
         var match = Regex.Match(size, regexPattern, RegexOptions.IgnoreCase);
 
         if (!match.Success)
         {
-            throw new ArgumentException("Invalid size format.", nameof(size));
+            throw new ArgumentException("Could not parse the provided value.", nameof(size));
         }
 
         var value = long.Parse(match.Groups["value"].Value);