diff --git a/src/Rules/PhpDoc/AssertRuleHelper.php b/src/Rules/PhpDoc/AssertRuleHelper.php
index 40551504fa..728a7f39b0 100644
--- a/src/Rules/PhpDoc/AssertRuleHelper.php
+++ b/src/Rules/PhpDoc/AssertRuleHelper.php
@@ -2,6 +2,7 @@
 
 namespace PHPStan\Rules\PhpDoc;
 
+use PHPStan\PhpDoc\Tag\AssertTag;
 use PHPStan\Node\Expr\TypeExpr;
 use PHPStan\Reflection\ExtendedMethodReflection;
 use PHPStan\Reflection\FunctionReflection;
@@ -20,7 +21,10 @@
 final class AssertRuleHelper
 {
 
-	public function __construct(private InitializerExprTypeResolver $initializerExprTypeResolver)
+	public function __construct(
+		private InitializerExprTypeResolver $initializerExprTypeResolver,
+		private UnresolvableTypeHelper $unresolvableTypeHelper,
+	)
 	{
 	}
 
@@ -55,9 +59,25 @@ public function check(ExtendedMethodReflection|FunctionReflection $reflection, P
 				continue;
 			}
 
+			if ($this->unresolvableTypeHelper->containsUnresolvableType($assert->getType())) {
+				$tagName = [
+					AssertTag::NULL => '@phpstan-assert',
+					AssertTag::IF_TRUE => '@phpstan-assert-if-true',
+					AssertTag::IF_FALSE => '@phpstan-assert-if-false',
+				][$assert->getIf()];
+
+				$errors[] = RuleErrorBuilder::message(sprintf(
+					'PHPDoc tag %s for parameter $%s contains unresolvable type.',
+					$tagName,
+					$parameterName,
+				))->identifier('parameter.unresolvableType')->build();
+
+				continue;
+			}
+
 			$assertedExpr = $assert->getParameter()->getExpr(new TypeExpr($parametersByName[$parameterName]));
 			$assertedExprType = $this->initializerExprTypeResolver->getType($assertedExpr, $context);
-			if ($assertedExprType instanceof ErrorType) {
+			if ($this->unresolvableTypeHelper->containsUnresolvableType($assertedExprType)) {
 				continue;
 			}
 
diff --git a/tests/PHPStan/Rules/PhpDoc/FunctionAssertRuleTest.php b/tests/PHPStan/Rules/PhpDoc/FunctionAssertRuleTest.php
index 3bfd8ae7a6..946528f150 100644
--- a/tests/PHPStan/Rules/PhpDoc/FunctionAssertRuleTest.php
+++ b/tests/PHPStan/Rules/PhpDoc/FunctionAssertRuleTest.php
@@ -54,6 +54,18 @@ public function testRule(): void
 				'Asserted negated type string for $i with type int does not narrow down the type.',
 				70,
 			],
+			[
+				'PHPDoc tag @phpstan-assert for parameter $str contains unresolvable type.',
+				83,
+			],
+			[
+				'PHPDoc tag @phpstan-assert-if-true for parameter $str contains unresolvable type.',
+				90,
+],
+			[
+				'PHPDoc tag @phpstan-assert-if-false for parameter $str contains unresolvable type.',
+				97,
+			],
 		]);
 	}
 
diff --git a/tests/PHPStan/Rules/PhpDoc/MethodAssertRuleTest.php b/tests/PHPStan/Rules/PhpDoc/MethodAssertRuleTest.php
index 932a09c299..cf5d540bb3 100644
--- a/tests/PHPStan/Rules/PhpDoc/MethodAssertRuleTest.php
+++ b/tests/PHPStan/Rules/PhpDoc/MethodAssertRuleTest.php
@@ -54,6 +54,18 @@ public function testRule(): void
 				'Asserted negated type string for $i with type int does not narrow down the type.',
 				72,
 			],
+			[
+				'PHPDoc tag @phpstan-assert for parameter $str contains unresolvable type.',
+				86,
+			],
+			[
+				'PHPDoc tag @phpstan-assert-if-true for parameter $str contains unresolvable type.',
+				93,
+],
+			[
+				'PHPDoc tag @phpstan-assert-if-false for parameter $str contains unresolvable type.',
+				100,
+			],
 		]);
 	}
 
diff --git a/tests/PHPStan/Rules/PhpDoc/data/function-assert.php b/tests/PHPStan/Rules/PhpDoc/data/function-assert.php
index e1b25eb6c0..494a47c20f 100644
--- a/tests/PHPStan/Rules/PhpDoc/data/function-assert.php
+++ b/tests/PHPStan/Rules/PhpDoc/data/function-assert.php
@@ -76,3 +76,24 @@ function negate1(int $i): void
 function negate2(int $i): void
 {
 }
+
+/**
+ * @phpstan-assert empty-str $str
+ */
+function unresolvableAssert(string $str): void
+{
+}
+
+/**
+ * @phpstan-assert-if-true empty-str $str
+ */
+function unresolvableAssertIfTrue(string $str): void
+{
+}
+
+/**
+ * @phpstan-assert-if-false empty-str $str
+ */
+function unresolvableAssertIfFalse(string $str): void
+{
+}
diff --git a/tests/PHPStan/Rules/PhpDoc/data/method-assert.php b/tests/PHPStan/Rules/PhpDoc/data/method-assert.php
index fa603eb85f..93cc988363 100644
--- a/tests/PHPStan/Rules/PhpDoc/data/method-assert.php
+++ b/tests/PHPStan/Rules/PhpDoc/data/method-assert.php
@@ -79,4 +79,26 @@ public function negate1(int $i): void
 	public function negate2(int $i): void
 	{
 	}
+
+	/**
+	 * @phpstan-assert empty-str $str
+	 */
+	public function unresolvableAssert(string $str): void
+	{
+	}
+
+	/**
+	 * @phpstan-assert-if-true empty-str $str
+	 */
+	public function unresolvableAssertIfTrue(string $str): void
+	{
+	}
+
+	/**
+	 * @phpstan-assert-if-false empty-str $str
+	 */
+	public function unresolvableAssertIfFalse(string $str): void
+	{
+	}
+
 }