diff --git a/src/Draco.Compiler.Tests/Draco.Compiler.Tests.csproj b/src/Draco.Compiler.Tests/Draco.Compiler.Tests.csproj
index 058508eed..ffa0b58a6 100644
--- a/src/Draco.Compiler.Tests/Draco.Compiler.Tests.csproj
+++ b/src/Draco.Compiler.Tests/Draco.Compiler.Tests.csproj
@@ -26,4 +26,10 @@
   <ItemGroup>
     <PackageReference Include="Basic.Reference.Assemblies.Net80" Version="1.4.5" />
   </ItemGroup>
+
+  <ItemGroup>
+    <None Update="Syntax\FuzzerTestCases\**">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
 </Project>
diff --git a/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-16a56ef2ee1f6a2fcc8ddf2911e0f3fb1b36e3b9.draco b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-16a56ef2ee1f6a2fcc8ddf2911e0f3fb1b36e3b9.draco
new file mode 100644
index 000000000..166595935
--- /dev/null
+++ b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-16a56ef2ee1f6a2fcc8ddf2911e0f3fb1b36e3b9.draco
@@ -0,0 +1 @@
+"\u{333333}
\ No newline at end of file
diff --git a/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-81bbb6ad94b3f4802afcaab9ef895fc0ddef70d4.draco b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-81bbb6ad94b3f4802afcaab9ef895fc0ddef70d4.draco
new file mode 100644
index 000000000..d4817db59
--- /dev/null
+++ b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/crash-81bbb6ad94b3f4802afcaab9ef895fc0ddef70d4.draco
@@ -0,0 +1 @@
+'󉉉���������������������������������������
diff --git a/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/timeout-0faaf5cf37107e7f0ff88777816a4b98f5fc3c63.draco b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/timeout-0faaf5cf37107e7f0ff88777816a4b98f5fc3c63.draco
new file mode 100644
index 000000000..a84e0db90
--- /dev/null
+++ b/src/Draco.Compiler.Tests/Syntax/FuzzerTestCases/timeout-0faaf5cf37107e7f0ff88777816a4b98f5fc3c63.draco
@@ -0,0 +1 @@
+߄
diff --git a/src/Draco.Compiler.Tests/Syntax/LexerTests.cs b/src/Draco.Compiler.Tests/Syntax/LexerTests.cs
index c2a456f02..3a32ae675 100644
--- a/src/Draco.Compiler.Tests/Syntax/LexerTests.cs
+++ b/src/Draco.Compiler.Tests/Syntax/LexerTests.cs
@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Text;
 using Draco.Compiler.Api.Diagnostics;
 using Draco.Compiler.Api.Syntax;
@@ -1277,4 +1278,25 @@ public void TestHelloWorld()
         this.AssertNextToken(TokenKind.Semicolon, ";");
         this.AssertNextToken(TokenKind.EndOfInput);
     }
+
+    [Theory]
+    [InlineData("timeout-0faaf5cf37107e7f0ff88777816a4b98f5fc3c63.draco")]
+    [InlineData("crash-16a56ef2ee1f6a2fcc8ddf2911e0f3fb1b36e3b9.draco")]
+    [InlineData("crash-81bbb6ad94b3f4802afcaab9ef895fc0ddef70d4.draco")]
+    public void TestFuzzerSampleTimeout(string path)
+    {
+        var text = File.ReadAllText(Path.Combine("Syntax", "FuzzerTestCases", path), Encoding.UTF8);
+        var source = SourceReader.From(text);
+        var lexer = new Lexer(source, this.diagnostics);
+        var stopwatch = Stopwatch.StartNew();
+        while (true)
+        {
+            var token = lexer.Lex();
+            if (token.Kind == TokenKind.EndOfInput) break;
+            if (stopwatch.ElapsedMilliseconds > 5000 && !Debugger.IsAttached)
+            {
+                Assert.Fail("Timeout.");
+            }
+        }
+    }
 }
diff --git a/src/Draco.Compiler/Internal/Syntax/Lexer.cs b/src/Draco.Compiler/Internal/Syntax/Lexer.cs
index 55af5e931..5f6ba98ac 100644
--- a/src/Draco.Compiler/Internal/Syntax/Lexer.cs
+++ b/src/Draco.Compiler/Internal/Syntax/Lexer.cs
@@ -244,7 +244,7 @@ Unit TakeWithText(TokenKind tokenKind, int length)
         // NOTE: We check for numeric literals first, so we can be lazy with the identifier checking later
         // Since digits would be a valid identifier character, we can avoid separating the check for the
         // first character
-        if (char.IsDigit(ch))
+        if (IsDigit(ch))
         {
             // Check for what kind of integer do we have
             var radix = this.Peek(1);
@@ -261,14 +261,14 @@ Unit TakeWithText(TokenKind tokenKind, int length)
             }
             var offset = 1;
             var isFloat = false;
-            while (char.IsDigit(this.Peek(offset))) ++offset;
+            while (IsDigit(this.Peek(offset))) ++offset;
 
             // Floating point number
-            if (this.Peek(offset) == '.' && char.IsDigit(this.Peek(offset + 1)))
+            if (this.Peek(offset) == '.' && IsDigit(this.Peek(offset + 1)))
             {
                 isFloat = true;
                 offset += 2;
-                while (char.IsDigit(this.Peek(offset))) ++offset;
+                while (IsDigit(this.Peek(offset))) ++offset;
             }
 
             if (char.ToLower(this.Peek(offset)) == 'e')
@@ -276,7 +276,7 @@ Unit TakeWithText(TokenKind tokenKind, int length)
                 isFloat = true;
                 ++offset;
                 if (this.Peek(offset) == '+' || this.Peek(offset) == '-') ++offset;
-                if (!char.IsDigit(this.Peek(offset)))
+                if (!IsDigit(this.Peek(offset)))
                 {
                     this.AddError(
                         template: SyntaxErrors.UnexpectedFloatingPointLiteralEnd,
@@ -287,7 +287,7 @@ Unit TakeWithText(TokenKind tokenKind, int length)
                         .SetText(this.Advance(offset).Span.ToString());
                     return default;
                 }
-                while (char.IsDigit(this.Peek(offset))) ++offset;
+                while (IsDigit(this.Peek(offset))) ++offset;
             }
 
             if (isFloat)
@@ -396,7 +396,20 @@ var _ when ident.Span.SequenceEqual("while") => TokenKind.KeywordWhile,
             {
                 // Regular character
                 ++offset;
-                resultChar = new(ch2);
+                if (Rune.IsValid(ch2))
+                {
+                    resultChar = new(ch2);
+                }
+                else
+                {
+                    // UnexpectedInput
+                    this.AddError(
+                        SyntaxErrors.IllegalCharacterLiteral,
+                        offset: offset,
+                        width: 1,
+                        args: (int)ch2);
+                    resultChar = DefaultCharacter;
+                }
             }
             else
             {
@@ -738,6 +751,15 @@ private Rune ParseEscapeSequence(int escapeStart, ref int offset)
                 ++offset;
                 if (length > 0)
                 {
+                    if (!Rune.IsValid(unicodeValue))
+                    {
+                        this.AddError(
+                            SyntaxErrors.IllegalUnicodeCodepoint,
+                            offset: escapeStart,
+                            width: offset - escapeStart,
+                            unicodeValue);
+                        return DefaultCharacter;
+                    }
                     return new(unicodeValue);
                 }
                 else
@@ -931,6 +953,7 @@ private bool TryPeek(int offset, out char result) =>
     private static bool IsIdent(char ch) => char.IsLetterOrDigit(ch) || ch == '_';
     private static bool IsSpace(char ch) => char.IsWhiteSpace(ch) && !IsNewline(ch);
     private static bool IsNewline(char ch) => ch == '\r' || ch == '\n';
+    private static bool IsDigit(char ch) => ch >= '0' && ch <= '9';
     private static bool TryParseHexDigit(char ch, out int value)
     {
         if (ch >= '0' && ch <= '9')
diff --git a/src/Draco.Compiler/Internal/Syntax/SyntaxErrors.cs b/src/Draco.Compiler/Internal/Syntax/SyntaxErrors.cs
index 31098fce7..d8e6c1195 100644
--- a/src/Draco.Compiler/Internal/Syntax/SyntaxErrors.cs
+++ b/src/Draco.Compiler/Internal/Syntax/SyntaxErrors.cs
@@ -46,6 +46,15 @@ internal static class SyntaxErrors
         format: "unclosed unicode codepoint escape sequence",
         code: Code(4));
 
+    /// <summary>
+    /// A \u{...} construct that represent an invalid codepoint.
+    /// </summary>
+    public static readonly DiagnosticTemplate IllegalUnicodeCodepoint = DiagnosticTemplate.Create(
+        title: "illegal unicode codepoint",
+        severity: DiagnosticSeverity.Error,
+        format: "illegal unicode codepoint (code: {0})",
+        code: Code(5));
+
     /// <summary>
     /// An illegal escape character after '\'.
     /// </summary>
@@ -53,7 +62,7 @@ internal static class SyntaxErrors
         title: "illegal escape character",
         severity: DiagnosticSeverity.Error,
         format: "illegal escape character '{0}'",
-        code: Code(5));
+        code: Code(6));
 
     /// <summary>
     /// A certain kind of token was expected while parsing.
@@ -62,7 +71,7 @@ internal static class SyntaxErrors
         title: "expected token",
         severity: DiagnosticSeverity.Error,
         format: "expected token {0}",
-        code: Code(6));
+        code: Code(7));
 
     /// <summary>
     /// Some kind of unexpected input while parsing.
@@ -71,7 +80,7 @@ internal static class SyntaxErrors
         title: "unexpected input",
         severity: DiagnosticSeverity.Error,
         format: "unexpected input while parsing {0}",
-        code: Code(7));
+        code: Code(8));
 
     /// <summary>
     /// Insufficient indentation in a multiline string.
@@ -80,7 +89,7 @@ internal static class SyntaxErrors
         title: "insufficient indentation",
         severity: DiagnosticSeverity.Error,
         format: "insufficient indentation in multiline string",
-        code: Code(8));
+        code: Code(9));
 
     /// <summary>
     /// There are extra tokens inline with the opening quotes of a multiline string.
@@ -89,7 +98,7 @@ internal static class SyntaxErrors
         title: "illegal tokens",
         severity: DiagnosticSeverity.Error,
         format: "illegal tokens inline with opening quotes of multiline string",
-        code: Code(9));
+        code: Code(10));
 
     /// <summary>
     /// The closing quotes of a multiline string are not on a new line.
@@ -98,7 +107,7 @@ internal static class SyntaxErrors
         title: "closing quotes are not on a new line",
         severity: DiagnosticSeverity.Error,
         format: "closing quotes are not on a new line of multiline string",
-        code: Code(10));
+        code: Code(11));
 
     /// <summary>
     /// The literal ended unexpectedly.
@@ -107,7 +116,7 @@ internal static class SyntaxErrors
         title: "unexpected floating-point literal end",
         severity: DiagnosticSeverity.Error,
         format: "unexpected end of scientific notation floating-point literal, expected one or more digits after exponent",
-        code: Code(11));
+        code: Code(12));
 
     /// <summary>
     /// The character literal ended unexpectedly.
@@ -116,7 +125,7 @@ internal static class SyntaxErrors
         title: "unexpected character literal end",
         severity: DiagnosticSeverity.Error,
         format: "unexpected end of character literal",
-        code: Code(12));
+        code: Code(13));
 
     /// <summary>
     /// The escape sequence ended unexpectedly.
@@ -125,7 +134,7 @@ internal static class SyntaxErrors
         title: "unexpected escape sequence end",
         severity: DiagnosticSeverity.Error,
         format: "unexpected end of escape sequence",
-        code: Code(13));
+        code: Code(14));
 
     /// <summary>
     /// An illegal language element in the context.
@@ -134,7 +143,7 @@ internal static class SyntaxErrors
         title: "illegal element in context",
         severity: DiagnosticSeverity.Error,
         format: "illegal language element {0} in context",
-        code: Code(14));
+        code: Code(15));
 
     /// <summary>
     /// There is a visibility modifier before an import statement.
@@ -143,5 +152,5 @@ internal static class SyntaxErrors
         title: "unexpected visibility modifier before import statement",
         severity: DiagnosticSeverity.Error,
         format: "unexpected {0} before import statement",
-        code: Code(15));
+        code: Code(16));
 }