This happens when trying to run `npm ls --json` at the build stage. If there are any missing peer dependencies, the command exits with a non-zero exit code. However, most of the time those errors can be safely ignored. An attempt to mitigate this is already in place: ``` def _get_installed_node_packages(self, cwd): # There is no yarn ls cmd = [os.path.join(self._npm_dir, "bin", "npm"), "ls", "--json"] try: output = self.run_output(cmd, cwd) except subprocess.CalledProcessError as error: # XXX When dependencies have missing dependencies, an error like # this is printed to stderr: # npm ERR! peer dep missing: glob@*, required by glob-promise@3.1.0 # retcode is not 0, which raises an exception. output = error.output.decode(sys.getfilesystemencoding()).strip() ``` However, `run_output()` itself captures the `CalledProcessError`, rasing a different type of exception (SnapcraftPluginCommandError): ``` def run_output(self, cmd, cwd=None, **kwargs): if not cwd: cwd = self.builddir os.makedirs(cwd, exist_ok=True) try: return common.run_output(cmd, cwd=cwd, **kwargs) except CalledProcessError as process_error: raise errors.SnapcraftPluginCommandError( command=cmd, part_name=self.name, exit_code=process_error.returncode ) from process_error ```