diff --git a/src/lib/codec-wrappers/codec.ts b/src/lib/codec-wrappers/codec.ts
index c99ea3a1..d0e632fb 100644
--- a/src/lib/codec-wrappers/codec.ts
+++ b/src/lib/codec-wrappers/codec.ts
@@ -1,5 +1,5 @@
 export interface Encoder {
-  encode(data: ImageData): Promise<ArrayBuffer | SharedArrayBuffer>;
+  encode(data: ImageData): Promise<ArrayBuffer>;
 }
 
 export interface Decoder {
diff --git a/src/lib/codec-wrappers/mozjpeg-enc.ts b/src/lib/codec-wrappers/mozjpeg-enc.ts
index f39fdaa6..0971ca73 100644
--- a/src/lib/codec-wrappers/mozjpeg-enc.ts
+++ b/src/lib/codec-wrappers/mozjpeg-enc.ts
@@ -57,7 +57,7 @@ export class MozJpegEncoder implements Encoder {
     })();
   }
 
-  async encode(data: ImageData): Promise<ArrayBuffer | SharedArrayBuffer> {
+  async encode(data: ImageData): Promise<ArrayBuffer> {
     const m = await this.emscriptenModule;
     const api = await this.api;
 
@@ -71,6 +71,7 @@ export class MozJpegEncoder implements Encoder {
     api.free_result();
     api.destroy_buffer(p);
 
-    return result.buffer;
+    // wasm can’t run on SharedArrayBuffers, so we hard-cast to ArrayBuffer.
+    return result.buffer as ArrayBuffer;
   }
 }