diff --git a/tools/check_for_upgrade.sh b/tools/check_for_upgrade.sh
index 33f30c85a..a36aecb84 100644
--- a/tools/check_for_upgrade.sh
+++ b/tools/check_for_upgrade.sh
@@ -95,13 +95,21 @@ function has_typed_input() {
   emulate -L zsh
   zmodload zsh/zselect
 
+  # Back up stty settings prior to disabling canonical mode
+  # Consider that no input can be typed if stty fails
+  # (this might happen if stdin is not a terminal)
+  local termios
+  termios=$(stty --save 2>/dev/null) || return 1
   {
-    local termios=$(stty --save)
+    # Disable canonical mode so that typed input counts
+    # regardless of whether Enter was pressed
     stty -icanon
 
+    # Poll stdin (fd 0) for data ready to be read
     zselect -t 0 -r 0
     return $?
   } always {
+    # Restore stty settings
     stty $termios
   }
 }