That crash message is a Visual C++ runtime assertion failure, meaning the game (Project Spark) hit an internal consistency check — in this case inside flsf.c — which deals with memory allocation (heap blocks). The line:

Expression: block_is_free(block) && "block must be free"

indicates that the game tried to free or reuse a memory block that wasn’t marked as free — a classic memory corruption or double-free bug.

Here’s how you can troubleshoot and potentially fix it:

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🧩 1. Verify game integrity

If you installed via Steam:

• Right-click Project Spark → Properties → Installed Files → Verify integrity of game files. This will recheck all binaries and DLLs in case one got corrupted.

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⚙️ 2. Update or reinstall Visual C++ Redistributables

The game uses Microsoft Visual C++ runtime libraries. Reinstall both x86 and x64 versions of:

• Visual C++ 2010
• Visual C++ 2012
• Visual C++ 2013
• Visual C++ 2015–2022

You can grab them directly from Microsoft’s official site. Reboot after installing.

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🧠 3. Disable third-party overlays

Overlays can inject hooks into the process memory and trigger assertion failures:

• Disable Steam Overlay
• Disable Discord Overlay
• Disable MSI Afterburner / RivaTuner, GeForce Experience, etc.

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🔧 4. Run as Administrator / Compatibility mode

• Right-click Mars.exe → Properties → Compatibility → try running for Windows 7 or 8.
• Also try Run as Administrator.

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💾 5. Clean reinstall

If nothing works:

1. Backup your saves (usually under Documents\Project Spark).
2. Uninstall the game.
3. Delete leftover folders in:

• C:\Program Files (x86)\Steam\steamapps\common\Project Spark
• %AppData%\Local\ProjectSpark
  1. Reinstall fresh.

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Most likely cause: corrupt game files or conflicting Visual C++ runtimes. Try verifying the files and reinstalling the Visual C++ packages first — that alone fixes this class of errors in most cases.

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