Reading environment variable in command alias causes evaluating it?

I have simple alias for writing to log files:

logtee() {
    ${TEST_LOG_FOLDER:?"empty_log_folder"}
    tee -a $TEST_LOG_FOLDER/$1
}

I'm reading the folder from environment variable. However, when I try using this alias by executing echo 'this is reading path from env.variable!' | logtee whatever.log it works, but the output is: sh.exe": /d/my/log/path: is a directory.
Did I do something wrong? Why does reading environment variable evaluate it? I suspect it have something to do with a fact that it's an alias, because in usual .sh script similar code works without any issues
P.S. I'm using msysgit on Windows 7, may be that's the problem?

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