dotnet/runtime · critical · Error

Failed to load ICU data

Error message

Failed to load ICU data

What it means

Thrown by wasm_load_icu_data when the underlying native call cwraps.wasm_load_icu_data(offset) returns a falsy value. It means the .NET WASM runtime failed to parse or install the provided ICU globalization data blob at the given memory offset.

Source

Thrown at src/mono/browser/runtime/icu.ts:9

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

import cwraps from "./cwraps";
import { VoidPtr } from "./types/emscripten";

export function wasm_load_icu_data (offset: VoidPtr) {
    if (!cwraps.wasm_load_icu_data(offset)) {
        throw new Error("Failed to load ICU data");
    }
}

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Use the ICU .dat file that ships with the exact .NET runtime version you are referencing (regenerate/republish assets).
2. Verify the ICU asset downloaded fully and matches its expected hash (check network/CORS and the asset config hash).
3. If you do not need globalization, configure invariant/globalization mode correctly so the runtime does not attempt to load ICU data.
4. Rebuild/publish the app to regenerate the correct blazor.boot.json / asset manifest with matching ICU data.

Example fix

// before: pointing config at a stale/mismatched icudt.dat
// dotnet.withConfig({ resources: { icu: [{ behavior:'icu', name:'icudt.dat', hash:'oldhash' }] } })

// after: republish so the asset manifest references the matching ICU file
// dotnet publish regenerates blazor.boot.json with the correct hash
// or set invariant mode if globalization is not needed:
// <InvariantGlobalization>true</InvariantGlobalization>

try {
  await createDotnetRuntime(opts);
} catch (e) {
  if (/ICU/i.test(String(e))) {
    // republish/restore the matching icudt.dat or enable invariant mode
  }
  throw e;
}

When it happens

Trigger: Produced during globalization initialization when the runtime hands an ICU .dat blob to the native loader and the native side rejects it (returns 0/null). Happens with a corrupt, truncated, version-mismatched, or wrong-architecture ICU .dat file.

Common situations: Shipping an ICU .dat built for a different endian/arch or a different .NET version; partial download of the ICU asset (network truncation, wrong hash); mixing invariant mode config with an ICU asset; mismatch between the runtime version and the ICU data file.

Related errors

• invariant globalization mode is inactive and no ICU data arc
• Locale or culture name is null or empty. localeName=${locale
• Locale info for locale=${localeName} is null or empty.
• Locale info for locale=${localeName} exceeds length of ${dst
• Failed to initialize ICU data

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/bd379d18db3369cc. Report an issue: GitHub.