diff --git a/package.json b/package.json
index a7804b8..0fdf020 100644
--- a/package.json
+++ b/package.json
@@ -58,7 +58,6 @@
     "ghooks": "^2.0.4",
     "globals": "^15.9.0",
     "in-publish": "^2.0.1",
-    "lodash.merge": "^4.6.2",
     "mocha": "^3.5.3",
     "npm-run-all": "^4.1.5",
     "nyc": "^11.9.0",
diff --git a/test/lib/rule-finder.js b/test/lib/rule-finder.js
index 5f0d42e..5345c43 100644
--- a/test/lib/rule-finder.js
+++ b/test/lib/rule-finder.js
@@ -4,13 +4,40 @@ const assert = require('assert');
 const proxyquire = require('proxyquire');
 const { builtinRules } = require('eslint/use-at-your-own-risk');
 const semver = require('semver');
-const merge = require('lodash.merge');
 const { ESLint } = require('eslint');
 
 const processCwd = process.cwd;
 const isV8Eslint = ESLint.configType === 'eslintrc';
 const eslintVersion = isV8Eslint ? 'prior-v8' : 'post-v8';
 
+const isObject = (item) => {
+  return item && typeof item === 'object' && !Array.isArray(item);
+};
+
+const deepMerge = (target, source) => {
+  if (!isObject(target)) {
+    target = {};
+  }
+  if (!isObject(source)) {
+    return target;
+  }
+
+  Object.keys(source).forEach(key => {
+    const sourceValue = source[key];
+    const targetValue = target[key];
+
+    if (Array.isArray(sourceValue)) {
+      target[key] = Array.isArray(targetValue) ? targetValue.concat(sourceValue) : sourceValue.slice();
+    } else if (isObject(sourceValue)) {
+      target[key] = isObject(targetValue) ? deepMerge(targetValue, sourceValue) : deepMerge({}, sourceValue);
+    } else {
+      target[key] = sourceValue;
+    }
+  });
+
+  return target;
+};
+
 const mockCreateRequire = (getExport, plugins, relative) => {
   // Use the mocked require.
   const moduleRequire = (id) => {
@@ -215,27 +242,16 @@ const getRuleFinder = isV8Eslint
             calculateConfigForFile(filePath) {
               const mergedConfig = {};
               defaultConfig.forEach(config => {
-                merge(mergedConfig, config);
+                deepMerge(mergedConfig, config);
               });
 
-              if(!specifiedFileRelative) {
-                if (Array.isArray(config)) {
-                  config.forEach(config => {
-                    merge(mergedConfig, config);
-                  });
-                  return mergedConfig;
-                } else {
-                  return merge(mergedConfig, config);
-                }
-              }
-
               if (Array.isArray(config)) {
                 config.forEach(config => {
-                  merge(mergedConfig, config);
+                  deepMerge(mergedConfig, config);
                 });
                 return mergedConfig;
               } else {
-                return merge(mergedConfig, config);
+                return deepMerge(mergedConfig, config);
               }
             }
           },
@@ -294,27 +310,16 @@ const getRuleFinderForDedupeTests = isV8Eslint
             calculateConfigForFile(filePath) {
               const mergedConfig = {};
               defaultConfig.forEach(config => {
-                merge(mergedConfig, config);
+                deepMerge(mergedConfig, config);
               });
 
-              if(!specifiedFileRelative) {
-                if (Array.isArray(config)) {
-                  config.forEach(config => {
-                    merge(mergedConfig, config);
-                  });
-                  return mergedConfig;
-                } else {
-                  return merge(mergedConfig, config);
-                }
-              }
-
               if (Array.isArray(config)) {
                 config.forEach(config => {
-                  merge(mergedConfig, config);
+                  deepMerge(mergedConfig, config);
                 });
                 return mergedConfig;
               } else {
-                return merge(mergedConfig, config);
+                return deepMerge(mergedConfig, config);
               }
             }
           },