diff --git a/src/Psalm/Checker/FileChecker.php b/src/Psalm/Checker/FileChecker.php
index d0c7df758..b76c95a87 100644
--- a/src/Psalm/Checker/FileChecker.php
+++ b/src/Psalm/Checker/FileChecker.php
@@ -404,7 +404,7 @@ class FileChecker extends SourceChecker implements StatementsSource
             throw new \UnexpectedValueException('Method ' . $method_id . ' could not be checked');
         }
 
-        foreach ($call_context->vars_in_scope as $var => $type) {
+        foreach ($call_context->vars_possibly_in_scope as $var => $_) {
             $this_context->vars_possibly_in_scope[$var] = true;
         }
 
diff --git a/tests/MethodMutationTest.php b/tests/MethodMutationTest.php
index f3cf6733a..5e7d0c002 100644
--- a/tests/MethodMutationTest.php
+++ b/tests/MethodMutationTest.php
@@ -102,6 +102,10 @@ class MethodMutationTest extends PHPUnit_Framework_TestCase
             public function barBar() : Response {
                 $this->setUser();
 
+                if (rand(0, 1)) {
+                    $this->title = "hello";
+                }
+
                 return new Response($this->user_viewdata);
             }
         }'
@@ -116,5 +120,6 @@ class MethodMutationTest extends PHPUnit_Framework_TestCase
 
         $this->assertEquals('UserViewData', (string)$method_context->vars_in_scope['$this->user_viewdata']);
         $this->assertEquals('string', (string)$method_context->vars_in_scope['$this->user_viewdata->name']);
+        $this->assertEquals(true, (string)$method_context->vars_possibly_in_scope['$this->title']);
     }
 }