Sarah Ericsson hung up the balloons in the shape of the number '21' for her birthday which resulted in her home in Karlskrona, Sweden, being visited by Swedish police who thought it stood for 'IS' and had been put up by supporters of the Islamic State extremist group.
"It was a little strange," the 21-year-old told The Local from the library at Blekinge Institute of Technology in southern Sweden, where she is taking a course in Spatial Planning.
A passerby had called them after looking through the window at the two balloons pushed together to make the number '21' and confusing their reverse image with the letters 'IS', which the extremist group also known as the Islamic State often uses as part of its propaganda.
"We understand why someone would report it if they thought it looked like IS-propaganda, although everyone else just thought it looked like the number '12' from outside," Ericsson said.
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When he went to open it, he was greeted by three police officers, who quickly realised that they had made a mistake.
"I laughed about it and they showed me a photo that they had taken where from their perspective, it did almost look like the letters IS," he told Swedish newspaper Kvallsposten.
But despite police accepting the error, Akesson said he was still asked to remove the balloons from the window.
"Extremism should always be taken seriously, and we did take the balloons down immediately," Ericsson said.
"I'm so surprised at all the attention. I will never forget my 21st birthday!" she added.