At the beginning of Harry Potter and The Deathly Hallows: Part 2 the gang tries to break into Bellatrix Lestrange's vault at Gringot's using polyjuice potion to dress Hermione and Ron as Bellatrix and some other Death Eater respectively. But when Hermione in Bellatrix costume is asked to hand over her wand for verification, she protests and the situation starts to get quite nervous and ultimately has to be resolved by a disguised Harry casting an Imperio curse upon the receptionist.
But I wonder why Hermione couldn't just hand over Bellatrix' wand as in the previous scene we were shown that they have it anyway, having it won at the end of Deathly Hallows: Part 1. Was this just to raise the tension and have it suprisingly resolved by Harry, who we at first didn't see was part of the crew? Or was there an actual story-wise reason that they couldn't just hand over Bellatrix' wand? (Or did they maybe just forget to bring it?) Is the source material clearer on this issue or does the movie even already give an explanation I just missed?