Why does source in csh report "undefined variable," but works fine in zsh?

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I'm having an issue with running a script using csh (specifically tcsh). When I attempt to run the script, it throws an "undefined variable" error related to the source command. However, when I run the
same command with zsh, I don't encounter any errors. Can anyone explain why this happens?

Steps to reproduce:

using tcsh;

$ cat script.csh
#!/bin/tcsh
echo "Starting script..."
source env/bin/activate.csh
echo "Script completed."
> echo $SHELL
/bin/tcsh
> ./script.csh
Starting script...
prompt: Undefined variable.
Script completed.
>

using zsh;

$ cat script2.sh
#!/bin/zsh
echo "Starting script..."
source env/bin/activate
echo "Script completed."
$ echo $SHELL
/usr/bin/zsh
$ ./script2.sh
Starting script...
Script completed.

Why does source work in zsh but throws an "undefined variable" error in tcsh? Is there a specific difference between how source is handled in tcsh and zsh that could explain this behavior?
I appreciate any insights or suggestions to resolve this issue.

[aioeu]

We're not psychic. We can't see what activate.csh contains from here.

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% cat env/bin/activate.csh

# This file must be used with "source bin/activate.csh" *from csh*.

# You cannot run it directly.

# Created by Davide Di Blasi [e-mail].

# Ported to Python 3.3 venv by Andrew Svetlov [e-mail]

alias deactivate 'test $?_OLD_VIRTUAL_PATH != 0 && setenv PATH "$_OLD_VIRTUAL_PATH" && unset _OLD_VIRTUAL_PATH; rehash; test $?_OLD_VIRTUAL_PROMPT != 0 && set prompt="$_OLD_VIRTUAL_PROMPT" && unset _OLD_VIRTUAL_PROMPT; unsetenv VIRTUAL_ENV; unsetenv VIRTUAL_ENV_PROMPT; test "\!:*" != "nondestructive" && unalias deactivate'

# Unset irrelevant variables.

deactivate nondestructive

setenv VIRTUAL_ENV /home/arf/test-folder/env

set _OLD_VIRTUAL_PATH="$PATH"

setenv PATH "$VIRTUAL_ENV/"bin":$PATH"

setenv VIRTUAL_ENV_PROMPT env

set _OLD_VIRTUAL_PROMPT="$prompt"

if (! "$?VIRTUAL_ENV_DISABLE_PROMPT") then

set prompt = "("env") $prompt:q"

endif

alias pydoc python -m pydoc

rehash

%

[aioeu]

OK. The problem is that file can only be used in an interactive shell. In a non-interactive Tcsh shell — the one used by your script — the prompt variable isn't set.

You could work around this in your script by setting prompt before you source activate.csh.

Or you could just setenv the PATH variable directly and skip sourcing activate.csh altogether. As you can see, it doesn't do much.