diff --git a/packages/ffmpeg/src/const.ts b/packages/ffmpeg/src/const.ts
index f01f071..1f97fe5 100644
--- a/packages/ffmpeg/src/const.ts
+++ b/packages/ffmpeg/src/const.ts
@@ -2,8 +2,8 @@ export const HeaderContentLength = "Content-Length";
 export const MIME_TYPE_JAVASCRIPT = "text/javascript";
 export const MIME_TYPE_WASM = "application/wasm";
 
-export const CORE_VERSION = "0.12.0";
-export const CORE_URL = `https://unpkg.com/@ffmpeg/core@${CORE_VERSION}/dist/ffmpeg-core.js`;
+export const CORE_VERSION = "0.12.0-alpha.2";
+export const CORE_URL = `https://unpkg.com/@ffmpeg/core@${CORE_VERSION}/dist/umd/ffmpeg-core.js`;
 
 export enum FFMessageType {
   LOAD = "load",
diff --git a/packages/ffmpeg/src/utils.ts b/packages/ffmpeg/src/utils.ts
index de13046..aa5e65e 100644
--- a/packages/ffmpeg/src/utils.ts
+++ b/packages/ffmpeg/src/utils.ts
@@ -33,23 +33,30 @@ export const downloadWithProgress = async (
     const reader = resp.body?.getReader();
     if (!reader) throw ERROR_RESPONSE_BODY_READER;
 
-    const data = new Uint8Array(total);
+    const chunks = [];
     let received = 0;
     for (;;) {
       const { done, value } = await reader.read();
       const delta = value ? value.length : 0;
 
       if (done) {
-        if (total !== received) throw ERROR_INCOMPLETED_DOWNLOAD;
+        if (total != -1 && total !== received) throw ERROR_INCOMPLETED_DOWNLOAD;
         cb({ url, total, received, delta, done });
         break;
       }
 
-      data.set(value, received);
+      chunks.push(value);
       received += delta;
       cb({ url, total, received, delta, done });
     }
 
+    const data = new Uint8Array(received);
+    let position = 0;
+    for (const chunk of chunks) {
+      data.set(chunk, position);
+      position += chunk.length;
+    }
+
     buf = data.buffer;
   } catch (e) {
     console.log(`failed to send download progress event: `, e);