The unidentified man entered the store in the city's Mitte district, placed a rucksack on the counter, produced a knife and started gesticulating wildly at the two female staff members, police said.
He then vaulted over the counter and continued a series of hand gestures that only further confused the women, aged 28 and 29.
"The offender said something unintelligible but the threatened shopkeeper did not respond," police said in a statement.
One ran into the street calling for help, prompting the man to flee after abandoning both knife and backpack, which the staff later realized he had wanted them to fill with cash from the register.
Police were searching for the would-be robber after Tuesday's incident which happened at 7pm.
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