Why does TMux fail to initialize oh-my-zsh correctly?

Question

Whenever I open a new TMux session, I'm greeted with this:

/home/foo/.oh-my-zsh/oh-my-zsh.sh:3: command not found: env
env_default:1: command not found: env
env_default:1: command not found: grep
env_default:1: command not found: env
env_default:1: command not found: grep

I've removed my .tmux.conf file and here's the lines from the oh-my-zsh.sh file:

  1 # Check for updates on initial load...                                                                                                                     
  2 if [ "$DISABLE_AUTO_UPDATE" != "true" ]; then
  3   env ZSH=$ZSH DISABLE_UPDATE_PROMPT=$DISABLE_UPDATE_PROMPT zsh -f $ZSH/tools/check_for_upgrade.sh
  4 fi
  5 

A regular terminal session does not have this problem...But, for some reason, this does...well, I should say, it doesn't display these errors. There may still be a problem that I don't know about. Now that I think about it, it has been awhile since oh-my-zsh asked to update itself.

Also, it seems like Vim is not loading .vimrc in Tmux either...Is this a PATH problem?

Any thoughts on what could be wrong here?

EDIT Here's my .tmux.conf

# remap prefix from 'C-b' to 'C-a'
unbind C-b
set-option -g prefix C-a
bind-key C-a send-prefix

# split panes using | and -
bind \ split-window -h
bind - split-window -v
unbind '"'
unbind %

# reload config file (change file location to your the tmux.conf you want to use)
#bind r source-file ~/.tmux.conf

# switch panes using Alt-arrow without prefix
bind h select-pane -L
bind l select-pane -R
bind k select-pane -U
bind j select-pane -D

# Enable mouse mode (tmux 2.1 and above)
set -g mouse on

#  Fix lag when switching modes in Vim
#set -sg escape-time 0

Answer 1

As mentioned in a comment by @jsonwryan:

Set the default-shell or default-command option in your .tmux.conf file:

set -g default-command /bin/zsh

(I'll never understand why people give answers in the comments instead of as answers)