Celtic failed to score for the second time during their German tour as they were beaten 1-0 by Stuttgart Kickers.
The Scottish champions drew their opening match of pre-season 0-0 with Augsburg in midweek and again struggled in front of goal against their third division opponents.
An experimental first-half line-up twice went close to an opening goal as Thomas Rogne headed over and Kris Commons lost his footing as he looked to chip the advancing keeper.
The Bhoys were not in any danger until two minutes before half-time when Andre Blackman slipped to allow winger Fabio Leutenecker to deliver for Omar Jatta to head home at the back post.
Hoops boss Neil Lennon made plenty of changes as the second-half wore on but Celtic's threat in front of goal became even more limited.
Dylan McGeouch was caught out by a long ball and Kevin Dicklhuber should have added a second goal, but he pulled his effort wide.
Sandrino Braun then got in front of Scott Brown but his shot grazed the crossbar and Lukasz Zaluska pulled off a good save in the last minute after Kelvin Wilson was turned inside the box as Celtic failed to find an equaliser.