diff --git a/Build/phpstan/phpstan-baseline.neon b/Build/phpstan/phpstan-baseline.neon
index 586c73e0..7206e73b 100644
--- a/Build/phpstan/phpstan-baseline.neon
+++ b/Build/phpstan/phpstan-baseline.neon
@@ -84,6 +84,12 @@ parameters:
 			count: 1
 			path: ../../src/Value/CSSFunction.php
 
+		-
+			message: '#^Function preg_match is unsafe to use\. It can return FALSE instead of throwing an exception\. Please add ''use function Safe\\preg_match;'' at the beginning of the file to use the variant provided by the ''thecodingmachine/safe'' library\.$#'
+			identifier: theCodingMachineSafe.function
+			count: 1
+			path: ../../src/Value/CSSString.php
+
 		-
 			message: '#^Function preg_match is unsafe to use\. It can return FALSE instead of throwing an exception\. Please add ''use function Safe\\preg_match;'' at the beginning of the file to use the variant provided by the ''thecodingmachine/safe'' library\.$#'
 			identifier: theCodingMachineSafe.function
@@ -102,6 +108,18 @@ parameters:
 			count: 3
 			path: ../../src/Value/Color.php
 
+		-
+			message: '#^Function preg_match is unsafe to use\. It can return FALSE instead of throwing an exception\. Please add ''use function Safe\\preg_match;'' at the beginning of the file to use the variant provided by the ''thecodingmachine/safe'' library\.$#'
+			identifier: theCodingMachineSafe.function
+			count: 1
+			path: ../../src/Value/Size.php
+
+		-
+			message: '#^Function preg_replace is unsafe to use\. It can return FALSE instead of throwing an exception\. Please add ''use function Safe\\preg_replace;'' at the beginning of the file to use the variant provided by the ''thecodingmachine/safe'' library\.$#'
+			identifier: theCodingMachineSafe.function
+			count: 2
+			path: ../../src/Value/Size.php
+
 		-
 			message: '#^Strict comparison using \!\=\= between non\-empty\-string and null will always evaluate to true\.$#'
 			identifier: notIdentical.alwaysTrue
diff --git a/src/Value/CSSString.php b/src/Value/CSSString.php
index 38d234ef..545a7ace 100644
--- a/src/Value/CSSString.php
+++ b/src/Value/CSSString.php
@@ -11,8 +11,6 @@
 use Sabberworm\CSS\Parsing\UnexpectedTokenException;
 use Sabberworm\CSS\ShortClassNameProvider;
 
-use function Safe\preg_match;
-
 /**
  * This class is a wrapper for quoted strings to distinguish them from keywords.
  *
@@ -58,7 +56,14 @@ public static function parse(ParserState $parserState): CSSString
         $result = '';
         if ($quote === null) {
             // Unquoted strings end in whitespace or with braces, brackets, parentheses
-            while (preg_match('/[\\s{}()<>\\[\\]]/isu', $parserState->peek()) === 0) {
+            while (true) {
+                $matchResult = \preg_match('/[\\s{}()<>\\[\\]]/isu', $parserState->peek());
+                if (!\is_int($matchResult)) {
+                    throw new \RuntimeException('The CSS is not valid UTF-8.', 1787548272);
+                }
+                if ($matchResult !== 0) {
+                    break;
+                }
                 $result .= $parserState->parseCharacter(false);
             }
         } else {
diff --git a/src/Value/Size.php b/src/Value/Size.php
index ad6d794c..addab489 100644
--- a/src/Value/Size.php
+++ b/src/Value/Size.php
@@ -10,9 +10,6 @@
 use Sabberworm\CSS\Parsing\UnexpectedTokenException;
 use Sabberworm\CSS\ShortClassNameProvider;
 
-use function Safe\preg_match;
-use function Safe\preg_replace;
-
 /**
  * A `Size` consists of a numeric `size` value and a unit.
  */
@@ -200,10 +197,19 @@ public function render(OutputFormat $outputFormat): string
     {
         $locale = \localeconv();
         $decimalPoint = \preg_quote($locale['decimal_point'], '/');
-        $size = preg_match('/[\\d\\.]+e[+-]?\\d+/i', (string) $this->size) === 1
-            ? preg_replace("/$decimalPoint?0+$/", '', \sprintf('%f', $this->size)) : (string) $this->size;
+        $matchResult = \preg_match('/[\\d\\.]+e[+-]?\\d+/i', (string) $this->size);
+        \assert(\is_int($matchResult));
+        if ($matchResult === 1) {
+            $size = \preg_replace("/$decimalPoint?0+$/", '', \sprintf('%f', $this->size));
+            \assert(\is_string($size));
+        } else {
+            $size = (string) $this->size;
+        }
+
+        $renderedSize = \preg_replace(["/$decimalPoint/", '/^(-?)0\\./'], ['.', '$1.'], $size);
+        \assert(\is_string($renderedSize));
 
-        return preg_replace(["/$decimalPoint/", '/^(-?)0\\./'], ['.', '$1.'], $size) . ($this->unit ?? '');
+        return $renderedSize . ($this->unit ?? '');
     }
 
     /**
diff --git a/tests/Unit/Value/CSSStringTest.php b/tests/Unit/Value/CSSStringTest.php
index 94196cfa..e1b4e519 100644
--- a/tests/Unit/Value/CSSStringTest.php
+++ b/tests/Unit/Value/CSSStringTest.php
@@ -6,6 +6,8 @@
 
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\OutputFormat;
+use Sabberworm\CSS\Parsing\ParserState;
+use Sabberworm\CSS\Settings;
 use Sabberworm\CSS\Value\CSSString;
 use Sabberworm\CSS\Value\PrimitiveValue;
 use Sabberworm\CSS\Value\Value;
@@ -144,4 +146,18 @@ public function doesNotEscapeDoubleQuotesThatDoNotNeedToBeEscaped(): void
 
         self::assertSame("'{$input}'", (new CSSString($input))->render($outputFormat));
     }
+
+    /**
+     * @test
+     */
+    public function parseThrowsForUnquotedStringThatIsNotValidUtf8WithoutMultibyteSupport(): void
+    {
+        $parserState = new ParserState("\xFF", Settings::create()->withMultibyteSupport(false));
+
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('The CSS is not valid UTF-8.');
+        $this->expectExceptionCode(1787548272);
+
+        CSSString::parse($parserState);
+    }
 }