Watch out for 6mm metric bits. They look like 1/4", but they're 1/3rd
of a mm too small and they won't clamp safely in a 1/4" router collet.
Actually the bit doesn't fits. I got a replacement of the same unbranded
kind of bit and it fits better

Alternatively, check you don't have a 6mm router collet and a 1/4"
bits.
Router is Makita 3606 1HP 1/4" collet. This problematic bit is being sold
without any brand. The stright bit branded "Irwin/American Tool" fits
perfectly. Whom trust? I think best to trust Makita...