diff --git a/src/offscreen/handlers/notary_prove.ts b/src/offscreen/handlers/notary_prove.ts
index cab74ffd..6bb637b1 100644
--- a/src/offscreen/handlers/notary_prove.ts
+++ b/src/offscreen/handlers/notary_prove.ts
@@ -196,12 +196,16 @@ async function calculateResponseSize(
 
     const contentLength = response.headers.get('content-length');
 
-    if (!contentLength) {
-        throw new Error('no content length in response headers');
+    let bodySize: number;
+
+    if (contentLength) {
+        bodySize = parseInt(contentLength, 10);
+    } else {
+        console.debug('fallback to measuring response blob due to no content-length header');
+        const blob = await response.blob();
+        bodySize = blob.size;
     }
 
-    const bodySize = parseInt(contentLength, 10);
-
     return headersSize + bodySize;
 }