diff --git a/lib/linter.js b/lib/linter.js
index 423df25..fc7c149 100644
--- a/lib/linter.js
+++ b/lib/linter.js
@@ -62,7 +62,7 @@ function lint(flowobj, config) {
         // Get the rule implementation
         let rule = resolveRule(ruleName);
 
-        if (!rule || config.rules[ruleName] === "off") {
+        if (!rule || !config.rules[ruleName] || config.rules[ruleName] === "off") {
             // Not a rule we know about, or rule disabled ("off")
             return
         }