What causes the DoAdd execution error in GameMaker?

The + operator received incompatible types — adding a string to a number (score displays, reading numbers from files or ini as strings) or an undefined value from a missed initialization.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix “DoAdd :: Execution Error” in GameMaker

Q: How do you fix the DoAdd execution error in GameMaker?

Convert explicitly at the operation (real()/string()), fix data loaded from files arriving as strings, and initialize variables before accumulating into them.

Quick answer: Convert explicitly at the operation (real()/string()), fix data loaded from files arriving as strings, and initialize variables before accumulating into them.

DoAdd is GML's way of saying 'I can't add these two things' — one side is a string or undefined when a number was expected. The classic sources are drawing code and file loading. Here is the sweep.

How to fix it

1. Fix the score-display pattern

draw_text(x, y, "Score: " + score) fails because score is a real; write "Score: " + string(score) — string concatenation demands strings on both sides.

2. Convert loaded data

ini_read_string, JSON string values, and text-file reads return strings; wrap with real() before math: hp += real(loaded_value).

3. Trace the undefined operand

The error text shows both operand values — undefined on one side means an uninitialized variable; find its declaration and give it a starting value.

4. Type-check at boundaries

For anything data-driven, validate with is_string()/is_real() when loading, so bad data fails at the loader with a clear message instead of in a Draw event.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every GameMaker error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Most of the time the fix is small. Seeing the failure clearly is the part that actually costs you.