Error: Could not prefetch mailalias provider 'aliases': Could not parse line "postmaster: root" (file: /etc/aliases, line: 3)

On a puppet agent run with agent 6.7.0 on Debian 9.9 "stretch", I get this error:

Error: Could not prefetch mailalias provider 'aliases': Could not parse line "postmaster: root" (file: /etc/aliases, line: 3)
Error: Failed to apply catalog: Could not parse line "postmaster: root" (file: /etc/aliases, line: 3)

This doesn't happen with agent 6.6.0.