Spot the mistake

I came across this rather fun little study:428357-man-leaning-on-his-guitar-case

This experiment tested the assumption that music plays a role in sexual selection. Three hundred young women were solicited in the street for their phone number by a young male confederate who held either a guitar case or a sports bag in his hands or had no bag at all. Results showed that holding a guitar case was associated with greater compliance to the request, thus suggesting that musical practice is associated with sexual selection. (Guéguen et al., see here).

Interesting conclusion – but is it justified?