Fix

assets/json/roman-numeral.json

@@ -11,5 +11,6 @@
 	"IX": 9,
 	"V": 5,
 	"IV": 4,
-	"I": 1
+	"I": 1,
+	"_nulla_": 0
 }

commands/number-edit/roman-numeral.js

@@ -14,6 +14,7 @@ module.exports = class RomanNumeralCommand extends Command {
 					key: 'number',
 					prompt: 'What number would you like to convert to roman numerals?',
 					type: 'integer',
+					min: 0,
 					max: 4999
 				}
 			]