WebAssembly New Added 12 September 2026

WebAssembly: null function or function signature mismatch

An indirect call went through the function table and found an empty slot, or a function whose type does not match the call site. In Emscripten builds it is a C function pointer cast to the wrong signature, or a JavaScript callback registered with a signature string that does not match what it is called with.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Rebuild with assertions: the release build reports only the trap
emcc app.c -o app.js -sASSERTIONS=2 -sSAFE_HEAP=1 -g

// addFunction signatures are positional: return type first, then arguments
// v void, i i32, j i64, f f32, d f64
const ptr = Module.addFunction((a, b) => a + b, 'iii');
Module.removeFunction(ptr);      // slots leak otherwise

// A callback stored across a module teardown points at a stale table
// Re-register after every instantiation rather than caching the pointer

# Casting function pointers is undefined behaviour in C and merely happens
# to work on native targets; fix the cast rather than papering over it
emcc app.c -o app.js -sEMULATE_FUNCTION_POINTER_CASTS=1   # slow, last resort

# Check the table entry the trap names
wasm-objdump -x app.wasm | grep -A5 'Elem\['

How to diagnose 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.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

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.
Validate the module offline with wasm-validate and inspect it with wasm-objdump -x from the WABT toolkit.
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.
Confirm imports match: every function the module imports must be supplied by the host with the exact name and signature, or instantiation fails.
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)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

emscripten.org

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