diff --git a/src/command.rs b/src/command.rs
index a5b6026..c872f2a 100644
--- a/src/command.rs
+++ b/src/command.rs
@@ -103,6 +103,8 @@ pub fn help(pargs: &mut Arguments) {
 }
 
 pub fn attach(pargs: &mut Arguments) {
+	util::prevent_nest();
+
 	let read_only = pargs.contains(["-r", "--readonly"]);
 	let detach_other = pargs.contains(["-d", "--detach"]);
 
@@ -204,6 +206,8 @@ pub fn list() {
 pub fn new(pargs: &mut Arguments) {
 	use pico_args::Error;
 
+	util::prevent_nest();
+
 	let target_dir: Result<String, Error> = pargs.value_from_str(["-t", "--target"]);
 
 	let args = pargs.clone().finish();
diff --git a/src/util.rs b/src/util.rs
index f72c654..842e1b6 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -1,3 +1,7 @@
+use std::{
+	env::var,
+	process::exit
+};
 
 use tmux_interface::{
 	Session, Sessions,
@@ -13,3 +17,11 @@ pub fn get_sessions() -> Option<Vec<Session>> {
 	Some(sessions.unwrap().0)
 }
 
+pub fn prevent_nest() {
+	let tmux = var("TMUX").ok();
+	if tmux.is_some() && tmux.unwrap() != "" {
+		println!("Sessions should be nested with care; unset TMUX to allow.");
+		exit(1);
+	}
+}
+