diff --git a/.eslintrc b/.eslintrc
index 5597fdc..44d150f 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -7,6 +7,7 @@
 		"consistent-return": 1,
 		"func-name-matching": 0,
 		"max-nested-callbacks": 0,
+		"max-lines-per-function": 0,
 		"sort-keys": 0,
 	},
 }
diff --git a/getLockfile.js b/getLockfile.js
index ca351bb..423cd43 100644
--- a/getLockfile.js
+++ b/getLockfile.js
@@ -23,6 +23,9 @@ module.exports = function getLockfile(packageFile, date, {
 	if (typeof date !== 'undefined' && !new Date(date).getTime()) {
 		return Promise.reject(colors.red(`\`date\` must be a valid Date format if provided; got ${inspect(date)}`));
 	}
+	if (only && only !== 'prod' && only !== 'production') {
+		return Promise.reject(colors.red('`only`, when provided, must be "prod" or "production"'));
+	}
 	const tmpDirP = getProjectTempDir({ npmNeeded, logger });
 	const npmRC = path.join(path.dirname(packageFile), '.npmrc');
 	const copyPkg = tmpDirP.then((tmpDir) => {