ISDone.dll / unarc.dll error “An error occurred when unpacking”: how to fix it
This message appears while installing a game, when the setup program cannot unpack its archives. ISDone.dll (an Inno Setup extension) and unarc.dll (the decompressor of the FreeArc archiver) report the failure, but are rarely its cause.
In short
- The problem almost always comes from disk space, memory or a damaged archive.
- Replacing unarc.dll or ISDone.dll only fixes the cases where these files are missing or blocked by the antivirus.
- Go through the checks below in order, then run the installation again.
The error message
It usually looks like this: “ISDone.dll: An error occurred when unpacking! Unarc.dll returned an error code: -1”, sometimes with another code (-7, -11, -12…) and the line “ERROR: archive data corrupted (decompression fails)”. Whatever the code, the unpacking stopped part of the way through.
The checks, in order
- Disk space: allow at least twice the size of the game on the installation drive, and a few GB free on the
C:drive (temporary files). - Installation folder: choose a short path, without accents or special characters, for example
C:\Games\GameName. - Virtual memory: Control Panel › System › Advanced system settings › Performance › Settings › Advanced › Virtual memory › Change, then check “Automatically manage paging file size for all drives”.
- Open programs: close the browser, launchers and resource-hungry software during the installation.
- Antivirus: check its quarantine; a single file blocked during unpacking is enough to make the installation fail.
- Damaged archive: if the error always occurs at the same percentage, download the setup program again.
- Run the setup program as administrator (right-click).
If the installation still fails
- Test the RAM with the Windows Memory Diagnostic tool (type mdsched in the Start menu): a faulty memory module often causes this kind of error.
- Check the drive: right-click the drive › Properties › Tools › Check.
- Try installing on another drive.
DLL files concerned
Each file page offers the download of the file alone and the complete method suited to it.
Frequently asked questions
Why does the error always occur at the same percentage?
It is the sign that part of the archive is damaged at that point: download the setup program again.
I have enough disk space, what should I do?
Check the virtual memory, test the RAM with Windows Memory Diagnostic and check the drive: these are the most frequent causes after a lack of space.