diff --git a/src/index.ts b/src/index.ts
index 53b8a20..6987b07 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -3,7 +3,7 @@ import mergeWith from "./merge-with";
 import joinArrays from "./join-arrays";
 import unique from "./unique";
 import { CustomizeRule, ICustomizeOptions, Key } from "./types";
-import { isPlainObject } from "./utils";
+import { isPlainObject, isUndefined } from "./utils";
 
 function merge<Configuration extends object>(
   firstConfiguration: Configuration | Configuration[],
@@ -22,9 +22,8 @@ function mergeWithCustomize<Configuration extends object>(
     firstConfiguration: Configuration | Configuration[],
     ...configurations: Configuration[]
   ): Configuration {
-    // No configuration at all
-    if (!firstConfiguration) {
-      return {} as Configuration;
+    if (isUndefined(firstConfiguration) || configurations.some(isUndefined)) {
+      throw new TypeError("Merging undefined is not supported");
     }
 
     // @ts-ignore
@@ -32,6 +31,11 @@ function mergeWithCustomize<Configuration extends object>(
       throw new TypeError("Promises are not supported");
     }
 
+    // No configuration at all
+    if (!firstConfiguration) {
+      return {} as Configuration;
+    }
+
     if (configurations.length === 0) {
       if (Array.isArray(firstConfiguration)) {
         // Empty array
@@ -39,6 +43,10 @@ function mergeWithCustomize<Configuration extends object>(
           return {} as Configuration;
         }
 
+        if (firstConfiguration.some(isUndefined)) {
+          throw new TypeError("Merging undefined is not supported");
+        }
+
         // @ts-ignore
         if (firstConfiguration[0].then) {
           throw new TypeError("Promises are not supported");
diff --git a/src/utils.ts b/src/utils.ts
index dea8b32..463506e 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -17,4 +17,8 @@ function isPlainObject(a) {
   return typeof a === "object";
 }
 
-export { isRegex, isFunction, isPlainObject };
+function isUndefined(a) {
+  return typeof a === "undefined";
+}
+
+export { isRegex, isFunction, isPlainObject, isUndefined };
diff --git a/test/merge.test.ts b/test/merge.test.ts
index da1aea8..5538680 100644
--- a/test/merge.test.ts
+++ b/test/merge.test.ts
@@ -19,8 +19,63 @@ function normalMergeTests(merge) {
 }
 
 function normalMergeTest(merge, loadersKey) {
-  it("should work with an empty configuration", function() {
-    assert.deepStrictEqual(merge(), {});
+  it("should throw with an empty configuration", function() {
+    assert.throws(() => merge(), {
+      name: "TypeError",
+      message: "Merging undefined is not supported"
+    });
+  });
+
+  it("should throw with undefined 1/4", function() {
+    assert.throws(() => merge(undefined), {
+      name: "TypeError",
+      message: "Merging undefined is not supported"
+    });
+  });
+
+  it("should throw with undefined 2/4", function() {
+    assert.throws(() => merge([undefined]), {
+      name: "TypeError",
+      message: "Merging undefined is not supported"
+    });
+  });
+
+  it("should throw with undefined 2/4", function() {
+    const result = { devServer: null };
+
+    assert.throws(
+      () =>
+        merge(
+          {
+            devServer: { base: true }
+          },
+          undefined,
+          result
+        ),
+      {
+        name: "TypeError",
+        message: "Merging undefined is not supported"
+      }
+    );
+  });
+
+  it("should throw with undefined 3/4", function() {
+    const result = { devServer: null };
+
+    assert.throws(
+      () =>
+        merge(
+          undefined,
+          {
+            devServer: { base: true }
+          },
+          result
+        ),
+      {
+        name: "TypeError",
+        message: "Merging undefined is not supported"
+      }
+    );
   });
 
   it("should work with an empty array", function() {