In command line help, we have

 -- Function File: strel ("disk", RADIUS)
 -- Function File: strel ("disk", RADIUS, N)
     Create disk shaped flat structuring element.  RADIUS must be a
     positive integer.

But in command window, we have an error message from the first input format.

strel('disk',1)
error: strel: N for disk shape not yet implemented, use N of 0
error: called from
    strel at line 257 column 9

This works:

strel('disk',1, 0)
ans =
  Flat STREL object with 5 neighbors

  Neighborhood:
  0  1  0
  1  1  1
  0  1  0