diff --git a/packages/web/src.ts/index.ts b/packages/web/src.ts/index.ts
index dd6eb83ca8..700f7698f1 100644
--- a/packages/web/src.ts/index.ts
+++ b/packages/web/src.ts/index.ts
@@ -154,6 +154,9 @@ export function _fetchData<T = Uint8Array>(connection: string | ConnectionInfo,
         if (headers["content-type"] == null) {
             headers["content-type"] = { key: "Content-Type", value: "application/octet-stream" };
         }
+        if (headers["content-length"] == null) {
+            headers["content-length"] = { key: "Content-Length", value: String(body.length) };
+        }
     }
 
     const flatHeaders: { [ key: string ]: string } = { };