diff --git a/src/config.ts b/src/config.ts
index 0974a75..14a363c 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -92,7 +92,10 @@ export function normalizeConfig(
     ignore: [...base.ignore],
   };
 
-  if (Array.isArray(obj.disabledRules)) {
+  if (obj.disabledRules !== undefined) {
+    if (!Array.isArray(obj.disabledRules)) {
+      throw new Error("Invalid disabledRules; expected an array of rule ids.");
+    }
     config.disabledRules = obj.disabledRules.map(String);
   }
   if (Array.isArray(obj.enabledRules)) {
diff --git a/test/config.test.ts b/test/config.test.ts
index 660d9ef..a5fad9b 100644
--- a/test/config.test.ts
+++ b/test/config.test.ts
@@ -36,6 +36,12 @@ describe("normalizeConfig", () => {
     );
   });
 
+  it("rejects a non-array disabledRules value", () => {
+    expect(() => normalizeConfig({ disabledRules: "MCP001" })).toThrow(
+      /disabledRules.*array/i,
+    );
+  });
+
   it("rejects an invalid severity override", () => {
     expect(() =>
       normalizeConfig({ severityOverrides: { MCP001: "nope" } }),