convert a:png is giving error. why?

[Sadiq3210]

convert a.png -crop 40x30+10+10 a40-3-0-10-10.png
is working fine. but

convert a: -crop 40x30+10+10 a40-3-0-10-10.png results in error

convert: must specify image size `' @ error/raw.c/ReadRAWImage/140.
convert: no images defined `a40-3-0-10-10.png' @ error/convert.c/ConvertImageCommand/3282.

[fmw42]

a: is not an image

[Sadiq3210]

I am trying to generate examples from this website https://www.imagemagick.org/Usage/crop/

convert rose:                    rose.gif   
convert rose: -crop 40x30+10+10  crop.gif   
convert rose: -crop 40x30+40+30 crop_br.gif   
convert rose: -crop 40x30-10-10  crop_tl.gif   
convert rose: -crop 90x60-10-10  crop_all.gif   
convert rose: -crop 40x30+90+60  crop_miss.gif

What does rose: implies here?

[fmw42]

rose: is an internal Imagemagick image. It includes the : and has no suffix. See https://imagemagick.org/script/formats.php#builtin-images All built-in images end in : No other image use the colon. They must have proper suffix.