ash / shell scripting question / abort after Arithmetic Evaluation

[Bastian Bittorf]

we stumbled over an interesting behaviour,
which is the same like in bash, but new to us:

root at box:~ a=120; echo $(( a / 60 )); echo HERE
2
HERE

root at box:~ a=; echo $(( a / 60 )); echo HERE
0
HERE

root at box:~ a=; echo $(( $a / 60 )); echo HERE
-ash: arithmetic syntax error

root at box:~ echo $(( / 60 )); echo HERE
-ash: arithmetic syntax error

can somebody explain, why the execution stopped
totally in the last 2 examples? this was the root-cause
for a not removed 'lock'-dir in one of our scripts...

bye, bastian