Inadvertant Whistle on Free throw

[kslcol]

I have a rules question that I need clarification on:

A1 has the ball for his second free throw attempt when B1, in a marked lane space, steps into the lane. The official inadvertantly sounds the whistle and stops play. Team B requests and is granted a timeout. Following the timeout, A1 misses the free throw. The official sounds the whistle and awards A1 a substitute free throw. Is the official correct?

Thanks in advance for any assistance.