diff --git a/thefuck/shells/bash.py b/thefuck/shells/bash.py
index c8f9f6a..52eb06e 100644
--- a/thefuck/shells/bash.py
+++ b/thefuck/shells/bash.py
@@ -29,7 +29,7 @@ class Bash(Generic):
                            if settings.alter_history else ''))
 
     def instant_mode_alias(self, alias_name):
-        if os.environ.get('THEFUCK_INSTANT_MODE'):
+        if os.environ.get('THEFUCK_INSTANT_MODE', '').lower() == 'true':
             return '''
                 export PS1="{user_command_mark}$PS1";
                 {app_alias}
diff --git a/thefuck/shells/zsh.py b/thefuck/shells/zsh.py
index 9d307fa..aafbd00 100644
--- a/thefuck/shells/zsh.py
+++ b/thefuck/shells/zsh.py
@@ -28,7 +28,7 @@ class Zsh(Generic):
                            if settings.alter_history else ''))
 
     def instant_mode_alias(self, alias_name):
-        if os.environ.get('THEFUCK_INSTANT_MODE'):
+        if os.environ.get('THEFUCK_INSTANT_MODE', '').lower() == 'true':
             return '''
                 export PS1="{user_command_mark}$PS1";
                 {app_alias}