Error 1721
There is a problem with this Windows Installer package. A program required for this install to complete could not be run.
The installer needed to run a helper program and could not launch it at all. That is a step earlier than error 1722, where the helper ran and then failed.
What it actually means
Custom actions launch external programs during installation. Error 1721 means the launch itself failed, usually because the executable was missing, blocked by security software, or could not be run with the permissions available.
How to fix it, in order
Ordered by how often each one is the answer, not by how clever it sounds.
- 01Run the installer as administrator, since custom actions frequently need elevation.
- 02Temporarily disable antivirus. Blocking a helper executable before it starts is the single most common cause of 1721 rather than 1722.
- 03Produce a verbose log with "msiexec /i package.msi /l*v install.log" and find the custom action named just before the failure.
- 04Check the prerequisites that action depends on, commonly a .NET or Visual C++ runtime.
- 05Try a clean boot, so no third-party service is interfering with process creation during the install.
Common questions
- What is the difference between error 1721 and 1722?
- In 1721 the installer could not start the helper program at all, usually blocked by security software or missing. In 1722 the helper started and then returned a failure, so the cause lies in what it was trying to do.
Before you retry the installer
If the package came from anywhere other than the publisher, confirm it is the file they actually shipped. Check its SHA-256 against our index, which settles in seconds whether you are debugging an installer or a damaged download.