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;
   }