I know this post is old but it led me to one bug report which led to another which led to errata about the "at" command being fixed for this issue on RHEL6.
However, as noted in the latter bug report which is at:
This is not just specific to the "at" command. Here we just ran across it when trying to relink Oracle on RHEL6.
The solution would be to unset this as noted in a prior post here but regular unset of a variable doesn't work because of the () at the end of BASH_FUNC(). The correct way to unset was in comment 47:
unset -f `env | sed -n 's/^BASH_FUNC_\([a-z]*\)()=.*/\1/p'`
Doing that on RHEL6.8 worked to do the unset.
Oops - On RHEL6.8 it did unset the variable but gave error before doing it:
-bash: unset: `–f': not a valid identifier
Apparently you don't need the "-f" - just the rest of the line:
unset `env | sed -n 's/^BASH_FUNC_\([a-z]*\)()=.*/\1/p'`
That will do it without the error but either way it does the unset.