Celtic failed to score for a second pre-season game as they fell to German third division side Stuttgart Kickers.
Neil Lennon's experimental first-half line-up conceded a headed goal just before the break and they struggled to carve out any second-half openings.
Their hosts had several chances to add to the lead in the final 20 minutes.
Celtic, who opened their tour with a goalless draw against Augsburg in midweek, started with a midfield diamond with Paddy McCourt playing in a deep central role.
The Northern Ireland international had Scott Brown and Filip Twardzik on either side with Kris Commons behind front pair Daryl Murphy and Gary Hooper.
Thomas Rogne and Charlie Mulgrew started at the back flanked by young full-backs Lewis Toshney and Andre Blackman with Lukasz Zaluska in goal.
Rogne missed a good chance with a header early on and Commons slipped as he tried to chip the goalkeeper and saw his effort sail over the crossbar.
Celtic did not look in any danger until two minutes before half-time when Blackman slipped to allow right-winger Fabio Leutenecker to cross for Omar Jatta to guide a free header home at the back post.
James Forrest, Beram Kayal and Kelvin Wilson came on at half-time with McCourt moving to the left wing and Celtic played a more square midfield four.
Forrest made a good early run that ended with a Commons shot being blocked but Celtic could not sustain any attacking threat.
Lennon brought on youngsters Dylan McGeouch, Tony Watt and Rabiu Ibrahim and Kickers created several late chances.
McGeouch, playing at right-back, was caught out by a ball over his shoulder but Kevin Dicklhuber pulled his shot wide.
Sandrino Braun got in front of Brown but his shot grazed the crossbar and Zaluska pulled off a good save in the last minute after Wilson had been turned inside the box.