Convert (numerically ordered) bitmap files to video file using avconv

I am attempting to convert a directory full of .bmp files into a .mp4 file (or similar format).

The bitmap files have the following name scheme:

output_N_1024.bmp

Where N is an integer in the range 0 to 1023. (No zero padding / fixed width.)

The command I am using is:

avconv -r 25 -i output_{0..1023}_1024.bmp outputfile.mp4

This appears to run okay, and takes about a minute to convert all 1024, 1024 by 1024 resolution - (confusing?) bitmap images into a new file, outputfile.mp4.

However, when I attempt to open this file with VLC, a black window briefly flashes up and then closes. VLC then goes back to its mode where it waits for you to tell it which file to open next. No error or warning messages appear from VLC, which seems kind of strange since it seems to be refusing to play.

What can I do to fix this? Perhaps my converting command is incorrect?