diff --git a/src/Mapping/Driver/SymfonyFileLocator.php b/src/Mapping/Driver/SymfonyFileLocator.php
index ff662d78..44626192 100644
--- a/src/Mapping/Driver/SymfonyFileLocator.php
+++ b/src/Mapping/Driver/SymfonyFileLocator.php
@@ -12,10 +12,8 @@
 
 use function array_keys;
 use function array_merge;
-use function assert;
 use function is_dir;
 use function is_file;
-use function is_int;
 use function realpath;
 use function sprintf;
 use function str_replace;
@@ -244,11 +242,14 @@ public function findMappingFile(string $className)
         }
 
         $pos = strrpos($className, '\\');
-        assert(is_int($pos));
 
         throw MappingException::mappingFileNotFound(
             $className,
-            substr($className, $pos + 1) . $this->fileExtension
+            // A class in the root namespace has no separator, and then its
+            // short name is the whole name: DateTime is DateTime, which is a
+            // perfectly good thing to name in the message rather than a
+            // reason to fail an assertion.
+            ($pos === false ? $className : substr($className, $pos + 1)) . $this->fileExtension
         );
     }
 
diff --git a/tests/Mapping/SymfonyFileLocatorTest.php b/tests/Mapping/SymfonyFileLocatorTest.php
index b04484ca..9b02e32f 100644
--- a/tests/Mapping/SymfonyFileLocatorTest.php
+++ b/tests/Mapping/SymfonyFileLocatorTest.php
@@ -189,6 +189,22 @@ public function testFindMappingFileNotFound(): void
         $locator->findMappingFile('Foo\\stdClass2');
     }
 
+    public function testFindMappingFileNotFoundForClassInRootNamespace(): void
+    {
+        $path   = __DIR__ . '/_files';
+        $prefix = 'Foo';
+
+        $locator = new SymfonyFileLocator([$path => $prefix], '.yml');
+
+        // A class with no namespace separator used to fail the assertion that
+        // computed the short name for this message, so with zend.assertions
+        // on this threw AssertionError instead of MappingException. Reported
+        // from \DateTime reaching here through AbstractQuery::setParameter().
+        $this->expectException(MappingException::class);
+        $this->expectExceptionMessage("No mapping file found named 'DateTime.yml' for class 'DateTime'.");
+        $locator->findMappingFile('DateTime');
+    }
+
     public function testFindMappingFileLeastSpecificNamespaceFirst(): void
     {
         // Low -> High