diff --git a/src/Psalm/Internal/Visitor/ReflectorVisitor.php b/src/Psalm/Internal/Visitor/ReflectorVisitor.php
index 93ddd716ad2..957dde1fdec 100644
--- a/src/Psalm/Internal/Visitor/ReflectorVisitor.php
+++ b/src/Psalm/Internal/Visitor/ReflectorVisitor.php
@@ -2440,11 +2440,18 @@ private function getAssertionParts(
         $assertion_type_parts = explode('|', $assertion_type);
 
         foreach ($assertion_type_parts as $i => $assertion_type_part) {
-            if ($assertion_type_part !== 'falsy'
-                && !isset($template_types[$assertion_type_part])
-                && !isset(Type::PSALM_RESERVED_WORDS[$assertion_type_part])
-            ) {
-                $assertion_type_parts[$i] = $prefix . Type::getFQCLNFromString($assertion_type_part, $this->aliases);
+            if ($assertion_type_part !== 'falsy') {
+                $namespaced_type = Type::parseTokens(
+                    Type::fixUpLocalType(
+                        $assertion_type_part,
+                        $this->aliases,
+                        $this->function_template_types + $this->class_template_types,
+                        $this->type_aliases,
+                        null
+                    )
+                );
+
+                $assertion_type_parts[$i] = $prefix . $namespaced_type->getId();
             } else {
                 $assertion_type_parts[$i] = $prefix . $assertion_type_part;
             }
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index 3ef15a8c1a0..f57ab027c60 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -908,6 +908,54 @@ function assertIntOrFoo($b) : void {
 
                     if (!is_int($a)) $a->bar();',
             ],
+            'assertUnionInNamespace' => [
+                '<?php
+                    namespace Foo\Bar\Baz;
+
+                    /**
+                      * @psalm-template ExpectedType of object
+                      * @param mixed $value
+                      * @psalm-param class-string<ExpectedType> $interface
+                      * @psalm-assert ExpectedType|class-string<ExpectedType> $value
+                      */
+                    function implementsInterface($value, $interface, string $message = ""): void {}
+
+                    /**
+                      * @psalm-template ExpectedType of object
+                      * @param mixed $value
+                      * @psalm-param class-string<ExpectedType> $interface
+                      * @psalm-assert null|ExpectedType|class-string<ExpectedType> $value
+                      */
+                    function nullOrImplementsInterface(?object $value, $interface, string $message = ""): void {}
+
+                    interface A
+                    {
+                    }
+
+                    /**
+                     * @param mixed $value
+                     *
+                     * @psalm-return A|class-string<A>
+                     */
+                    function consume($value) {
+                        implementsInterface($value, A::class);
+
+                        return $value;
+                    }
+
+
+                    /**
+                     * @param mixed $value
+                     *
+                     * @psalm-return A|class-string<A>|null
+                     */
+                    function consume2($value)
+                    {
+                        nullOrImplementsInterface($value, A::class);
+
+                        return $value;
+                    }'
+            ],
         ];
     }