[FFmpeg-user] "Invalid sample rate" and "Guessed Channel Layout" warnings from -i /dev/zero

[gheine at mathnmaps.com]

Goal is to create a silent 4-second audio that can be merged with a 
static 4-second video to create a simple menu for a DVD (NTSC format).  
Entering

ffmpeg -f s16le -i /dev/zero -ac 2  -ab 224k -ar 48000 -t 4 -acodec ac3 
-y tmp.ac3

gives the warnings

[s16le @ 0xaf96560] Invalid sample rate 0 specified using default of 
44100
[s16le @ 0xaf96560] Estimating duration from bitrate, this may be 
inaccurate
Guessed Channel Layout for  Input Stream #0.0 : mono

However, running ffplay on the output yields:

   Duration: 00:00:04.00, start: 0.000000, bitrate: 224 kb/s
     Stream #0:0: Audio: ac3, 48000 Hz, stereo, s16, 224 kb/s

Which looks exactly right.
So, what is causing the warnings?
Does this have something to do with the input being a stream of binary 
zeroes?