diff --git a/src/Tracing/SpanStack.php b/src/Tracing/SpanStack.php
index 02f7cf2..226c23c 100644
--- a/src/Tracing/SpanStack.php
+++ b/src/Tracing/SpanStack.php
@@ -19,7 +19,7 @@ class SpanStack
     /**
      * @var Span[]
      */
-    private $spans;
+    private $spans = [];
     /**
      * @var Tracer
      */
@@ -127,4 +127,4 @@ public static function get()
         }
         return $result;
     }
-}
\ No newline at end of file
+}