diff --git a/src/Visitor/DetectorVisitor.php b/src/Visitor/DetectorVisitor.php
index 62c5560..d8ab3bc 100644
--- a/src/Visitor/DetectorVisitor.php
+++ b/src/Visitor/DetectorVisitor.php
@@ -52,6 +52,9 @@ public function enterNode(Node $node): ?int
         if ($this->hasSign($node)) {
             $node = $node->getAttribute('parent');
             if ($this->isMinus($node)) {
+                if (!isset($scalar->value)) {
+                    return null;
+                }
                 $scalar->value = -$scalar->value;
             }
         }