Netlogo - Ordered Movement -

i have following error in netlogo , i'm unsure why. trying make turtles move around in ordered fashion keep getting error when change d angle: "face expected input agent got nobody instead". appreciated.

globals [angles] setup   clear-all   create-turtles amount [ setxy random-xcor random-ycor ]    ask turtles [   set angles (random 360)  ]    reset-ticks end  monitor  show count patches       show ticks end  go    if (all? patches [pcolor = yellow]) [stop]   ask turtles [      face min-one-of patches [ pcolor = black ] [ distance myself ]     ;; line of code tells turtle head towards nearest patch containing colour of black.     set angle  d  angle * 1 - angle     rightt angle     forwardd 1         ifelse show-travel-line? [pen-down][pen-up]     set color red       if pcolor = black [       set pcolor yellow        ]    ]  tick end 

you can unveil problem running test:

to test   ca   crt 1   let x -10e307 * 10   show x   ask turtle 0 [rt x]   inspect turtle 0 end 

you see heading nan because gave turn of -infinity. if move turtle, xcor , ycor become nan.

to avoid problem, need limit values taken angle. example,

globals [turn angle]  setup   clear-all   set turn random-float 1   create-turtles 10 [     setxy random-xcor random-ycor     set color red     pen-down   ]   reset-ticks end  go   if (all? patches [pcolor = yellow]) [stop]   ask turtles [     part1     part2     if pcolor = black [       set pcolor yellow     ]   ]   tick end  part1     let _patches (patches [ pcolor = black ])     face min-one-of _patches [ distance myself ] end  part2     set turn (4 * turn * (1 - turn))     set angle turn * 360     rt angle     fd 1 end