diff --git a/src/Psalm/Type.php b/src/Psalm/Type.php
index d83eaffce..e4063bdb6 100644
--- a/src/Psalm/Type.php
+++ b/src/Psalm/Type.php
@@ -944,6 +944,12 @@ abstract class Type
                 continue;
             }
 
+            if ($string_type_token[0][0] === '\\'
+                && strlen($string_type_token[0]) === 1
+            ) {
+                throw new TypeParseTreeException("Backslash \"\\\" has to be part of class name.");
+            }
+
             if ($string_type_token[0][0] === '"'
                 || $string_type_token[0][0] === '\''
                 || $string_type_token[0] === '0'
diff --git a/tests/AnnotationTest.php b/tests/AnnotationTest.php
index 2e29e3d51..1686e58f2 100644
--- a/tests/AnnotationTest.php
+++ b/tests/AnnotationTest.php
@@ -1195,6 +1195,16 @@ class AnnotationTest extends TestCase
                     }',
                 'error_message' => 'InvalidDocblock - src' . DIRECTORY_SEPARATOR . 'somefile.php:5:21 - Badly-formatted @param',
             ],
+            'invalidSlashWithString' => [
+                '<?php
+                    /**
+                     * @return \?string
+                     */
+                    function foo() {
+                      return rand(0, 1) ? "hello" : null;
+                    }',
+                'error_message' => 'InvalidDocblock',
+            ],
             'missingReturnTypeWithBadDocblock' => [
                 '<?php
                     /**