diff --git a/codecs/jxl/dec/jxl_dec.d.ts b/codecs/jxl/dec/jxl_dec.d.ts
index c2721da9..8fe96424 100644
--- a/codecs/jxl/dec/jxl_dec.d.ts
+++ b/codecs/jxl/dec/jxl_dec.d.ts
@@ -1,5 +1,5 @@
 interface JXLModule extends EmscriptenWasm.Module {
-  decode(data: BufferSource): ImageData;
+  decode(data: BufferSource): ImageData | null;
 }
 
-export default function(opts: EmscriptenWasm.ModuleOpts): JXLModule;
+export default function(opts: EmscriptenWasm.ModuleOpts): Promise<JXLModule>;
diff --git a/codecs/jxl/enc/jxl_enc.d.ts b/codecs/jxl/enc/jxl_enc.d.ts
index 39a5b63c..9e45a01f 100644
--- a/codecs/jxl/enc/jxl_enc.d.ts
+++ b/codecs/jxl/enc/jxl_enc.d.ts
@@ -1,8 +1,7 @@
-import { EncodeOptions } from '../../src/codecs/jxl/encoder-meta';
+import { EncodeOptions } from '../../../src/codecs/jxl/encoder-meta';
 
 interface JXLModule extends EmscriptenWasm.Module {
-  encode(data: BufferSource, width: number, height: number, options: EncodeOptions): Uint8Array;
+  encode(data: BufferSource, width: number, height: number, options: EncodeOptions): Uint8Array | null;
 }
 
-
-export default function(opts: EmscriptenWasm.ModuleOpts): JXLModule;
+export default function(opts: EmscriptenWasm.ModuleOpts): Promise<JXLModule>;
diff --git a/src/codecs/jxl/decoder.ts b/src/codecs/jxl/decoder.ts
index c7628856..997ebae2 100644
--- a/src/codecs/jxl/decoder.ts
+++ b/src/codecs/jxl/decoder.ts
@@ -8,5 +8,9 @@ export async function decode(data: ArrayBuffer): Promise<ImageData> {
   if (!emscriptenModule) emscriptenModule = initEmscriptenModule(jxl_dec, wasmUrl);
 
   const module = await emscriptenModule;
-  return module.decode(data);
+  const result = module.decode(data);
+  if (!result) {
+    throw new Error('Decoding error');
+  }
+  return result;
 }
diff --git a/src/codecs/jxl/encoder.ts b/src/codecs/jxl/encoder.ts
index a06e21b0..7a5ceb93 100644
--- a/src/codecs/jxl/encoder.ts
+++ b/src/codecs/jxl/encoder.ts
@@ -10,6 +10,9 @@ export async function encode(data: ImageData, options: EncodeOptions): Promise<A
 
   const module = await emscriptenModule;
   const result = module.encode(data.data, data.width, data.height, options);
+  if (!result) {
+    throw new Error('Encoding error');
+  }
 
   // wasm can’t run on SharedArrayBuffers, so we hard-cast to ArrayBuffer.
   return result.buffer as ArrayBuffer;