Error #2109 half the time - actionscript-3

Question

I'm making a platform game in AS3.
Everything is running fine except sometimes (not each time I run the game), I've got this error :
ArgumentError: Error #2109: Frame label null not found in scene null.
at flash.display::MovieClip/gotoAndStop()
at PlatformGame/moveCharacter()[C:\...\PlatformGame.as:581]
at PlatformGame/moveEnemies()[C:\...\PlatformGame.as:360]
at PlatformGame/gameLoop()[C:\...\PlatformGame.as:348]
I don't understand AT ALL why it happens randomly..
Do you know what could be the problem ?
Here's the lines in the error :
line 581 :
char.mc.gotoAndStop(char.walkAnimation[Math.floor(char.animstep)]);
line 360 :
moveCharacter(enemies[i],timeDiff);
line 348 :
moveEnemies(timeDiff);
Thank you for your help,

Answer 1

You ask to 'mc' (contained in 'char') to go to the frame which number
is contained in the array 'walkAnimation' at the index: Math.floor(char.animstep).
The error you have got means that, for some reason, 'Math.floor(char.animstep)' index doesn't exist in the array 'walkAnimation'.
Exactly as if you wrote :
var frame:Array = [1, 2, 3];
gotoAndStop(frame[10]);
frame[0] is 1, frame[1] is 2, frame[2] is 3, but frame[10] is undefined.