From 25e9ebcd51b2b82680e1a6eb35d2b0609f10a7ab Mon Sep 17 00:00:00 2001
From: Phil Pluckthun <phil@kitten.sh>
Date: Sun, 1 Feb 2026 22:31:35 +0000
Subject: [PATCH] fix: Protect against missing `Symbol.toStringTag` (#19)

---
 .changeset/wild-beers-repeat.md | 5 +++++
 src/body.ts                     | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/wild-beers-repeat.md

diff --git a/.changeset/wild-beers-repeat.md b/.changeset/wild-beers-repeat.md
new file mode 100644
index 0000000..906b006
--- /dev/null
+++ b/.changeset/wild-beers-repeat.md
@@ -0,0 +1,5 @@
+---
+'fetch-nodeshim': patch
+---
+
+Protect against missing `Symbol.toStringTag`
diff --git a/src/body.ts b/src/body.ts
index 65f8086..e1001b3 100644
--- a/src/body.ts
+++ b/src/body.ts
@@ -77,8 +77,8 @@ const isBlob = (object: any): object is Blob => {
     typeof object.stream === 'function' &&
     typeof object.constructor === 'function'
   ) {
-    const tag = object[Symbol.toStringTag];
-    return tag.startsWith('Blob') || tag.startsWith('File');
+    const tag = object[Symbol.toStringTag] as string | undefined;
+    return !!tag && (tag.startsWith('Blob') || tag.startsWith('File'));
   } else {
     return false;
   }
-- 
2.51.2