WebAssembly Errors

Understanding WebAssembly errors

WebAssembly errors are precise about the layer that failed. A CompileError means the bytes are not a valid module: most often the file was served with the wrong MIME type, or an HTML error page was fetched instead of the .wasm file. A RuntimeError: memory access out of bounds means the module read or wrote outside its linear memory, which in a language like C or Rust is a genuine memory-safety bug caught by the sandbox.

How to debug WebAssembly errors

1. Check what the server actually returned. The wasm file must be served as application/wasm for streaming compilation, and a 404 HTML page produces a misleading magic-number error.
2. Validate the module offline with wasm-validate and inspect it with wasm-objdump -x from the WABT toolkit.
3. For out-of-bounds errors, rebuild with sanitizers or debug assertions in the source language. The wasm runtime cannot tell you which source line was responsible without DWARF info.
4. Confirm imports match: every function the module imports must be supplied by the host with the exact name and signature, or instantiation fails.
5. Build with debug symbols and use the browser's DWARF support to step through original source rather than raw wasm.

Tools worth reaching for

• wasm-validate / wasm-objdump (WABT)
• browser DWARF debugging
• wasmtime --invoke
• curl -I (check MIME type)

All 17 WebAssembly errors

• Emscripten: Cannot call unknown function (make sure it is exported) Newccall and cwrap look the symbol up in the module at runtime, and the linker had already removed it because no C code…
• Emscripten: RuntimeError: Aborted(). Build with -sASSERTIONS for more info. NewEmscripten's runtime hit a fatal condition and called abort(), but release builds leave out assertions to save size…
• Rust: Could not find specification for target "wasm32-wasi" NewRust renamed the wasm32-wasi target to wasm32-wasip1 to make room for WASI 0.2, then removed the old name entirely in…
• wasm-bindgen: closure invoked after being dropped NewA Rust closure handed to JavaScript lives only as long as its Closure value, and letting that fall out of scope frees…
• wasm-bindgen: recursive use of an object detected NewA Rust method holding a mutable borrow of a struct called into JavaScript, and that JavaScript called back into the…
• wasm-pack: crate-type must be cdylib to compile to wasm32-unknown-unknown NewA Rust library defaults to the rlib crate type, which cannot be linked into a WebAssembly module, so wasm-pack stops…
• WebAssembly: Cannot enlarge memory arrays NewThe module asked for more linear memory than it was built to have. Emscripten fixes the heap at INITIAL_MEMORY unless…
• WebAssembly: CompileError - invalid magic / sectionThe .wasm file is not valid WebAssembly bytes. Often the server returned an HTML error page, the file was truncated…
• WebAssembly: Import #0 module="env" error: module is not an object or function NewThe module declares imports and the importObject handed to instantiate has no property for that namespace, so the…
• WebAssembly: incorrect response MIME type (expected application/wasm) NewinstantiateStreaming requires the response to be served as application/wasm. Servers that do not know the extension…
• WebAssembly: LinkError: import object field is not a Function NewInstantiation failed because the import object handed to the module does not match what the module declares it needs…
• WebAssembly: null function or function signature mismatch NewAn indirect call went through the function table and found an empty slot, or a function whose type does not match the…
• WebAssembly: Refused to compile or instantiate WebAssembly module NewThe page's Content Security Policy blocks WebAssembly compilation. Browsers treat compiling Wasm much like eval, so a…
• WebAssembly: RuntimeError - memory access out of boundsWASM code accessed memory outside its linear memory, usually from an out-of-bounds index, use-after-free, or…
• WebAssembly: RuntimeError: table index is out of bounds NewAn indirect call used a function pointer that is not in the module table, which normally means a callback was passed…
• WebAssembly: RuntimeError: unreachable executed NewThe module reached an `unreachable` instruction, which compilers emit wherever the program traps: a Rust panic, an…
• WebAssembly: SharedArrayBuffer is not defined NewThreaded builds need shared memory, and browsers only expose SharedArrayBuffer to cross origin isolated pages. The…

Other categories

• AI 35Rate limits, context windows, GPU memory and model-serving failures.
• Ansible 18Unreachable hosts, become passwords, undefined variables and Jinja2 failures.
• API 20Auth headers, payload limits, versioning, idempotency and webhook signatures.
• Apple 20Command line tools, dyld, Homebrew permissions, notarisation and Keychain.
• Auth 20OIDC, SAML, Auth0, Okta, Keycloak, passkeys and MFA failures.
• BigData 17Spark, Kafka, Airflow, Snowflake, Flink and Databricks failures.
• C# 18NuGet restore, null references, EF Core migrations, async deadlocks and Blazor…
• C++ 20Segfaults, linker errors, memory corruption and template deduction failures.
• Caching 20Cache stampedes, stale content, Varnish and CloudFront failures.
• CI/CD 18GitHub Actions, GitLab CI, Jenkins, CircleCI: permissions, runners and…
• Client 21CORS, mixed content, module resolution, memory limits and framework runtime…
• Cloud 25IAM permissions, quotas, service limits and credential failures.
• Dart 20Null safety, pub version solving and build toolchain problems.
• Database 41Connections, deadlocks, constraints, replication and memory limits.
• DNS 20NXDOMAIN, SERVFAIL, timeouts, propagation and delegation problems.
• Docker 27Daemon connectivity, disk space, image pulls, ports and architecture mismatches.
• Elixir 18GenServer timeouts, supervision failures and Mix compilation problems.
• Email 20Delivery failures, relay denial, authentication, SPF, DKIM and DMARC.
• Frontend 23Hydration mismatches, bundler resolution, layout shift and font loading.
• Git 20Merge conflicts, rejected pushes, detached HEAD, LFS and repository corruption.
• Go 19Nil map assignment, concurrent map access, context cancellation and deadlocks.
• GraphQL 19Validation, depth limits, N+1 queries and fragment problems.
• gRPC 17Status codes, deadlines, message limits, TLS and HTTP/2 transport failures.
• HTTP 494xx client errors, 5xx server errors, redirects, headers and protocol problems.
• ICMP 23Destination unreachable, time exceeded, fragmentation needed and redirects.
• Ingress 20404 default backend, missing TLS secrets, IngressClass and path matching.
• Java 19Class loading, dependency resolution, connection pools and JVM version…
• JavaScript 42npm resolution, async pitfalls, hydration, memory limits and runtime type…
• Kubernetes 34CrashLoopBackOff, ImagePullBackOff, OOMKilled, RBAC, scheduling and storage.
• Logging 18Log4j, Logback, Fluentd, Logstash and CloudWatch ingestion problems.
• MessageQueue 20Kafka, RabbitMQ, SQS, NATS and Celery: lag, rebalancing and poison messages.
• Mobile 20Gradle, CocoaPods, Xcode signing, Metro bundler and toolchain mismatches.
• Monitoring 18Prometheus scrapes, Grafana data sources, OpenTelemetry exporters and agent…
• Network 35Refused connections, timeouts, resets, MTU problems and port exhaustion.
• Performance 19GC pauses, thread pool starvation and event loop blocking.
• PHP 20Memory limits, execution timeouts, autoloading, Composer and PDO connections.
• Proxy 20nginx, Envoy, HAProxy, Traefik, Caddy and Cloudflare upstream failures.
• Python 35Imports, virtual environments, encoding, concurrency and dependency conflicts.
• Regex 17Catastrophic backtracking, back references, escaping and engine differences.
• Ruby 20Bundler, migrations, native extensions, encoding and asset compilation.
• Rust 19Borrow checker, ownership moves, trait bounds and lifetime mismatches.
• Scala 17Dependency resolution, binary compatibility and type inference failures.
• Security 25JWT validation, CSRF, OAuth grants, SELinux, SSH host keys and CSP.
• Serverless 20Lambda timeouts, package size limits, VPC networking and cold starts.
• Shell 19Command not found, permissions, quoting, expansion and Makefile syntax.
• Storage 19S3 permissions, NFS mounts, quotas, signed URLs and volume attachment.
• Svelte 20Store subscriptions, load functions and server/client boundaries.
• System 26Disk space, systemd units, file descriptors, OOM killer and scheduled jobs.
• Terraform 18State locks, provider auth, drift, dependency cycles and plan-time unknowns.
• Testing 18Jest, pytest, JUnit, Cypress and Playwright: fixtures, snapshots and timeouts.
• TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.
• TypeScript 19Assignability, missing declarations, strict null checks and generic constraints.
• Virtualization 20VirtualBox, VMware, Hyper-V, WSL, KVM and hypervisor conflicts.
• Web3 17Gas estimation, nonce management and reverted transactions.
• WebServer 20nginx, Apache, IIS and Caddy: binding, permissions, rewrites and TLS.
• Windows 20Installer failures, missing runtimes, update errors and permission problems.