AMD processors aren’t affected by the devastating Meltdown CPU flaw, but the emergency fix for Meltdown and Spectre can apparently bring certain AMD CPUs to their knees. Microsoft has stopped offering the Windows security patch to some AMD systems after reports of PCs not booting.

“After investigating, Microsoft has determined that some AMD chipsets do not conform to the documentation previously provided to Microsoft to develop the Windows operating system mitigations to protect against the chipset vulnerabilities known as Spectre and Meltdown,” Microsoft’s security advisory reads. “To prevent AMD customers from getting into an unbootable state, Microsoft will temporarily pause sending the following Windows operating system updates to devices with impacted AMD processors at this time.”

Microsoft doesn’t specify which AMD CPUs are vulnerable to the error. CSO reports that forum complaints largely revolve around older Athlon and Sempron chips, with the largest thread bearing over 160 replies and more than 800 “I have the same question” votes.

AMD and Microsoft are working to resolve the issue. Security updates for affected computers will resume after it’s fixed. If your AMD PC refuses to boot after installing last week’s emergency patch, Microsoft says its guides for troubleshooting bluescreen errors in Windows 7, 8, and 10 may help.

Even if you can’t install the Meltdown patch at this time, upgrade your web browser pronto. Firefox, Chrome, Internet Explorer, and Edge have all been updated with initial protections against the Spectre CPU exploit—a separate issue from Meltdown and these flawed Windows patches. AMD chips are susceptible to Spectre. Also be sure to keep your security software vigilant while you’re working without the Windows patch. It won’t be able to detect if the Meltdown or Spectre exploits are abused on your system, but an attacker needs to be able to inject and run malicious code on your PC to trigger the CPU flaws.