diff --git a/src/Analyser/SpecifiedTypes.php b/src/Analyser/SpecifiedTypes.php
index ccb2c5e8a36..cbaa5180107 100644
--- a/src/Analyser/SpecifiedTypes.php
+++ b/src/Analyser/SpecifiedTypes.php
@@ -5,12 +5,16 @@
 use PhpParser\Node\Expr;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
+use PHPStan\Type\UnionType;
 use function array_key_exists;
 use function array_merge;
+use function count;
 
 final class SpecifiedTypes
 {
 
+	private const MAX_ADDED_TYPES = 8;
+
 	private bool $overwrite = false;
 
 	/** @var array<string, ConditionalExpressionHolder[]> */
@@ -168,6 +172,10 @@ public function unionWith(SpecifiedTypes $other): self
 				continue;
 			}
 
+			if ($type instanceof UnionType && count($type->getTypes()) >= self::MAX_ADDED_TYPES) {
+				continue;
+			}
+
 			$sureTypeUnion[$exprString] = [
 				$exprNode,
 				TypeCombinator::intersect($type, $other->sureTypes[$exprString][1]),
@@ -179,6 +187,10 @@ public function unionWith(SpecifiedTypes $other): self
 				continue;
 			}
 
+			if ($type instanceof UnionType && count($type->getTypes()) >= self::MAX_ADDED_TYPES) {
+				continue;
+			}
+
 			$sureNotTypeUnion[$exprString] = [
 				$exprNode,
 				TypeCombinator::union($type, $other->sureNotTypes[$exprString][1]),
diff --git a/tests/PHPStan/Analyser/AnalyserIntegrationTest.php b/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
index 4e7036c6862..f9dfbbc5203 100644
--- a/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
+++ b/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
@@ -1632,6 +1632,12 @@ public function testBug13980(): void
 		$this->assertNoErrors($errors);
 	}
 
+	public function testBug14207(): void
+	{
+		$errors = $this->runAnalyse(__DIR__ . '/data/bug-14207.php');
+		$this->assertNoErrors($errors);
+	}
+
 	public function testBug13945(): void
 	{
 		$errors = $this->runAnalyse(__DIR__ . '/data/bug-13945.php');
diff --git a/tests/PHPStan/Analyser/data/bug-14207.php b/tests/PHPStan/Analyser/data/bug-14207.php
new file mode 100644
index 00000000000..a378443b790
--- /dev/null
+++ b/tests/PHPStan/Analyser/data/bug-14207.php
@@ -0,0 +1,153 @@
+<?php declare(strict_types = 1);
+
+namespace Bug14207;
+
+use function assert;
+use function PHPStan\Testing\assertType;
+
+class WP_HTML_Token {
+	public string $namespace;
+	public string $node_name;
+}
+
+class HelloWorld
+{
+	/**
+	 * Returns whether an element of a given name is in the HTML special category.
+	 *
+	 * @since 6.4.0
+	 *
+	 * @see https://html.spec.whatwg.org/#special
+	 *
+	 * @param WP_HTML_Token|string $tag_name Node to check, or only its name if in the HTML namespace.
+	 * @return bool Whether the element of the given name is in the special category.
+	 */
+	public static function is_special( $tag_name ): bool {
+		if ( is_string( $tag_name ) ) {
+			$tag_name = strtoupper( $tag_name );
+		} else {
+			$tag_name = 'html' === $tag_name->namespace
+				? strtoupper( $tag_name->node_name )
+				: "{$tag_name->namespace} {$tag_name->node_name}";
+		}
+
+		assertType('non-falsy-string|uppercase-string', $tag_name);
+
+		$x = (
+			'ADDRESS' === $tag_name ||
+			'APPLET' === $tag_name ||
+			'AREA' === $tag_name ||
+			'ARTICLE' === $tag_name ||
+			'ASIDE' === $tag_name ||
+			'BASE' === $tag_name ||
+			'BASEFONT' === $tag_name ||
+			'BGSOUND' === $tag_name ||
+			'BLOCKQUOTE' === $tag_name ||
+			'BODY' === $tag_name ||
+			'BR' === $tag_name ||
+			'BUTTON' === $tag_name ||
+			'CAPTION' === $tag_name ||
+			'CENTER' === $tag_name ||
+			'COL' === $tag_name ||
+			'COLGROUP' === $tag_name ||
+			'DD' === $tag_name ||
+			'DETAILS' === $tag_name ||
+			'DIR' === $tag_name ||
+			'DIV' === $tag_name ||
+			'DL' === $tag_name ||
+			'DT' === $tag_name ||
+			'EMBED' === $tag_name ||
+			'FIELDSET' === $tag_name ||
+			'FIGCAPTION' === $tag_name ||
+			'FIGURE' === $tag_name ||
+			'FOOTER' === $tag_name ||
+			'FORM' === $tag_name ||
+			'FRAME' === $tag_name ||
+			'FRAMESET' === $tag_name ||
+			'H1' === $tag_name ||
+			'H2' === $tag_name ||
+			'H3' === $tag_name ||
+			'H4' === $tag_name ||
+			'H5' === $tag_name ||
+			'H6' === $tag_name ||
+			'HEAD' === $tag_name ||
+			'HEADER' === $tag_name ||
+			'HGROUP' === $tag_name ||
+			'HR' === $tag_name ||
+			'HTML' === $tag_name ||
+			'IFRAME' === $tag_name ||
+			'IMG' === $tag_name ||
+			'INPUT' === $tag_name ||
+			'KEYGEN' === $tag_name ||
+			'LI' === $tag_name ||
+			'LINK' === $tag_name ||
+			'LISTING' === $tag_name ||
+			'MAIN' === $tag_name ||
+			'MARQUEE' === $tag_name ||
+			'MENU' === $tag_name ||
+			'META' === $tag_name ||
+			'NAV' === $tag_name ||
+			'NOEMBED' === $tag_name ||
+			'NOFRAMES' === $tag_name ||
+			'NOSCRIPT' === $tag_name ||
+			'OBJECT' === $tag_name ||
+			'OL' === $tag_name ||
+			'P' === $tag_name ||
+			'PARAM' === $tag_name ||
+			'PLAINTEXT' === $tag_name ||
+			'PRE' === $tag_name ||
+			'SCRIPT' === $tag_name ||
+			'SEARCH' === $tag_name ||
+			'SECTION' === $tag_name ||
+			'SELECT' === $tag_name ||
+			'SOURCE' === $tag_name ||
+			'STYLE' === $tag_name ||
+			'SUMMARY' === $tag_name ||
+			'TABLE' === $tag_name ||
+			'TBODY' === $tag_name ||
+			'TD' === $tag_name ||
+			'TEMPLATE' === $tag_name ||
+			'TEXTAREA' === $tag_name ||
+			'TFOOT' === $tag_name ||
+			'TH' === $tag_name ||
+			'THEAD' === $tag_name ||
+			'TITLE' === $tag_name ||
+			'TR' === $tag_name ||
+			'TRACK' === $tag_name ||
+			'UL' === $tag_name ||
+			'WBR' === $tag_name ||
+			'XMP' === $tag_name ||
+
+			// MathML.
+			'math MI' === $tag_name ||
+			'math MO' === $tag_name ||
+			'math MN' === $tag_name ||
+			'math MS' === $tag_name ||
+			'math MTEXT' === $tag_name ||
+			'math ANNOTATION-XML' === $tag_name ||
+
+			// SVG.
+			'svg DESC' === $tag_name ||
+			'svg FOREIGNOBJECT' === $tag_name ||
+			'svg TITLE' === $tag_name ||
+
+			// some random stuff
+			'a1' === $tag_name ||
+			'a2' === $tag_name ||
+			'a3' === $tag_name ||
+			'a4' === $tag_name ||
+			'a5' === $tag_name ||
+			'a6' === $tag_name ||
+			'a7' === $tag_name ||
+			'a8' === $tag_name ||
+			'a9' === $tag_name
+		);
+
+		assertType('bool', $x);
+		if ($x) {
+			assertType("'a1'|'a2'|'a3'|'a4'|'a5'|'a6'|'a7'|'a8'|'a9'|'ADDRESS'|'APPLET'|'AREA'|'ARTICLE'|'ASIDE'|'BASE'|'BASEFONT'|'BGSOUND'|'BLOCKQUOTE'|'BODY'|'BR'|'BUTTON'|'CAPTION'|'CENTER'|'COL'|'COLGROUP'|'DD'|'DETAILS'|'DIR'|'DIV'|'DL'|'DT'|'EMBED'|'FIELDSET'|'FIGCAPTION'|'FIGURE'|'FOOTER'|'FORM'|'FRAME'|'FRAMESET'|'H1'|'H2'|'H3'|'H4'|'H5'|'H6'|'HEAD'|'HEADER'|'HGROUP'|'HR'|'HTML'|'IFRAME'|'IMG'|'INPUT'|'KEYGEN'|'LI'|'LINK'|'LISTING'|'MAIN'|'MARQUEE'|'math ANNOTATION-XML'|'math MI'|'math MN'|'math MO'|'math MS'|'math MTEXT'|'MENU'|'META'|'NAV'|'NOEMBED'|'NOFRAMES'|'NOSCRIPT'|'OBJECT'|'OL'|'P'|'PARAM'|'PLAINTEXT'|'PRE'|'SCRIPT'|'SEARCH'|'SECTION'|'SELECT'|'SOURCE'|'STYLE'|'SUMMARY'|'svg DESC'|'svg FOREIGNOBJECT'|'svg TITLE'|'TABLE'|'TBODY'|'TD'|'TEMPLATE'|'TEXTAREA'|'TFOOT'|'TH'|'THEAD'|'TITLE'|'TR'|'TRACK'|'UL'|'WBR'|'XMP'", $tag_name);
+		}
+
+		return $x;
+	}
+}