diff --git a/lib/rule-tester/flat-rule-tester.js b/lib/rule-tester/flat-rule-tester.js
index 510cbc688ce..97055d104f4 100644
--- a/lib/rule-tester/flat-rule-tester.js
+++ b/lib/rule-tester/flat-rule-tester.js
@@ -345,7 +345,7 @@ class FlatRuleTester {
      * @returns {void}
      */
     static setDefaultConfig(config) {
-        if (typeof config !== "object") {
+        if (typeof config !== "object" || config === null) {
             throw new TypeError("FlatRuleTester.setDefaultConfig: config must be an object");
         }
         sharedDefaultConfig = config;
diff --git a/lib/rule-tester/rule-tester.js b/lib/rule-tester/rule-tester.js
index 48df3b79b94..8518299d0b0 100644
--- a/lib/rule-tester/rule-tester.js
+++ b/lib/rule-tester/rule-tester.js
@@ -412,7 +412,7 @@ class RuleTester {
      * @returns {void}
      */
     static setDefaultConfig(config) {
-        if (typeof config !== "object") {
+        if (typeof config !== "object" || config === null) {
             throw new TypeError("RuleTester.setDefaultConfig: config must be an object");
         }
         defaultConfig = config;
diff --git a/tests/lib/rule-tester/flat-rule-tester.js b/tests/lib/rule-tester/flat-rule-tester.js
index bdc196e1653..c73099d1d54 100644
--- a/tests/lib/rule-tester/flat-rule-tester.js
+++ b/tests/lib/rule-tester/flat-rule-tester.js
@@ -142,12 +142,14 @@ describe("FlatRuleTester", () => {
                     FlatRuleTester.setDefaultConfig(config);
                 };
             }
-            assert.throw(setConfig());
-            assert.throw(setConfig(1));
-            assert.throw(setConfig(3.14));
-            assert.throw(setConfig("foo"));
-            assert.throw(setConfig(null));
-            assert.throw(setConfig(true));
+            const errorMessage = "FlatRuleTester.setDefaultConfig: config must be an object";
+
+            assert.throw(setConfig(), errorMessage);
+            assert.throw(setConfig(1), errorMessage);
+            assert.throw(setConfig(3.14), errorMessage);
+            assert.throw(setConfig("foo"), errorMessage);
+            assert.throw(setConfig(null), errorMessage);
+            assert.throw(setConfig(true), errorMessage);
         });
 
         it("should pass-through the globals config to the tester then to the to rule", () => {
diff --git a/tests/lib/rule-tester/rule-tester.js b/tests/lib/rule-tester/rule-tester.js
index e961dd9a516..096e639e95f 100644
--- a/tests/lib/rule-tester/rule-tester.js
+++ b/tests/lib/rule-tester/rule-tester.js
@@ -1412,12 +1412,14 @@ describe("RuleTester", () => {
                 RuleTester.setDefaultConfig(config);
             };
         }
-        assert.throw(setConfig());
-        assert.throw(setConfig(1));
-        assert.throw(setConfig(3.14));
-        assert.throw(setConfig("foo"));
-        assert.throw(setConfig(null));
-        assert.throw(setConfig(true));
+        const errorMessage = "RuleTester.setDefaultConfig: config must be an object";
+
+        assert.throw(setConfig(), errorMessage);
+        assert.throw(setConfig(1), errorMessage);
+        assert.throw(setConfig(3.14), errorMessage);
+        assert.throw(setConfig("foo"), errorMessage);
+        assert.throw(setConfig(null), errorMessage);
+        assert.throw(setConfig(true), errorMessage);
     });
 
     it("should pass-through the globals config to the tester then to the to rule", () => {