A Linux kernel developer is suing VMware in Germany, alleging the company has not complied with copyright terms for using open-source software.

Christoph Hellwig, who holds copyrights on portions of the Linux kernel, alleges VMware combined proprietary source code with open-source code in its ESXi product line but has not released it publicly as required by the General Public License version 2 (GPLv2). The suit was filed in district court in Hamburg.

The Software Freedom Conservancy, a charity that supports open-source software projects, is funding Hellwig’s lawsuit through a grant, according to a news release.

VMware said Thursday it believes the lawsuit is without merit.

“VMware embraces, participates in and is committed to the open source community,” the company said. “We believe we will prevail on all issues through the judicial process in Germany.”

VMware is accused of wrapping its “vmkernel,” which is part of its ESXi virtualization software for servers, with open-source code. The vmkernel is an operating system that manages hardware resurces such as memory, processors, storage and networking controllers.

Hellwig also accused VMware of not complying with the GPLv2 for its version of BusyBox, a bundle of utilities incorporated in its ESXi products.

The Software Conservancy negotiated with VMware from late 2011 through 2013 over its concerns, according to a Q and A from the organization. VMware made “substantial and good efforts” to comply with BusyBox but didn’t fix a few minor problems and one major Linux compliance issue, which is the focus of the lawsuit, it said.