diff --git a/src/core/merge.js b/src/core/merge.js
index 7867a0b..17b39d8 100644
--- a/src/core/merge.js
+++ b/src/core/merge.js
@@ -1,54 +1,76 @@
+const assert = require('assert');
 const objectScan = require('object-scan');
 
+const last = (arr) => arr[arr.length - 1];
+const mkChild = (ref) => {
+  if (!(ref instanceof Object)) {
+    return ref;
+  }
+  return Array.isArray(ref) ? [] : {};
+};
+const populate = (obj, key, fn) => {
+  if (!(key in obj)) {
+    // eslint-disable-next-line no-param-reassign
+    obj[key] = fn();
+    return true;
+  }
+  return false;
+};
+const incompatible = (a, b) => (
+  !(a instanceof Object)
+  || !(b instanceof Object)
+  || Array.isArray(a) !== Array.isArray(b)
+);
+
 module.exports = (logic_ = {}) => {
   const logic = { '**': null, ...logic_ };
-  const last = (arr) => arr[arr.length - 1];
-  const mkChild = (ref) => {
-    if (!(ref instanceof Object)) {
-      return ref;
-    }
-    return Array.isArray(ref) ? [] : {};
-  };
-  const populate = (obj, key, fn, force = false) => {
-    if (force === true || !(key in obj)) {
-      // eslint-disable-next-line no-param-reassign
-      obj[key] = fn();
-      return true;
-    }
-    return false;
-  };
 
   const scanner = objectScan(Object.keys(logic), {
     reverse: false,
     breakFn: ({
       isMatch, property, value, matchedBy, context
     }) => {
-      if (!isMatch) return;
       const { stack, groups, path } = context;
       const current = last(stack);
+
+      if (!isMatch) {
+        if (incompatible(current, value)) {
+          stack[0] = mkChild(value);
+        }
+        return false;
+      }
+      if (!(current instanceof Object)) {
+        stack.push(null);
+        return true;
+      }
+      if (!Array.isArray(current)) {
+        if (!(property in current) || incompatible(current[property], value)) {
+          current[property] = mkChild(value);
+        }
+        stack.push(current[property]);
+        return false;
+      }
+
       const bestNeedle = last(matchedBy);
       const groupBy = typeof logic[bestNeedle] === 'function'
         ? logic[bestNeedle](value)
         : logic[bestNeedle];
 
-      if (!Array.isArray(current) || groupBy === null) {
-        if (Array.isArray(current)) {
-          current.push(mkChild(value));
-          stack.push(last(current));
-        } else {
-          populate(current, property, () => mkChild(value), !(value instanceof Object));
-          stack.push(current[property]);
-        }
-      } else {
-        const groupId = `${bestNeedle}.${groupBy}: ${path.join('.')}`;
-        populate(groups, groupId, () => ({}));
-        const groupEntryId = value[groupBy];
-        if (populate(groups[groupId], groupEntryId, () => mkChild(value))) {
-          current.push(groups[groupId][groupEntryId]);
-        }
-        path.push(`${groupBy}=${groupEntryId}`);
-        stack.push(groups[groupId][groupEntryId]);
+      if (groupBy === null) {
+        current.push(value);
+        stack.push(null);
+        return true;
+      }
+
+      const groupId = `${bestNeedle}.${groupBy}: ${path.join('.')}`;
+      populate(groups, groupId, () => ({}));
+      const groupEntryId = value instanceof Object ? value[groupBy] : undefined;
+      if (populate(groups[groupId], groupEntryId, () => mkChild(value))) {
+        current.push(groups[groupId][groupEntryId]);
       }
+      path.push(`${groupBy}=${groupEntryId}`);
+      stack.push(groups[groupId][groupEntryId]);
+      return false;
     },
     filterFn: ({ matchedBy, context }) => {
       const { stack, path } = context;
@@ -59,9 +81,10 @@ module.exports = (logic_ = {}) => {
     }
   });
   return (...args) => {
-    const result = mkChild(last(args));
+    const stack = [undefined];
     const groups = {};
-    args.forEach((arg) => scanner(arg, { stack: [result], groups, path: [] }));
-    return result;
+    args.forEach((arg) => scanner(arg, { stack, groups, path: [] }));
+    assert(stack.length === 1);
+    return stack[0];
   };
 };
diff --git a/test/core/merge.spec.js b/test/core/merge.spec.js
index 0d80f02..890a100 100644
--- a/test/core/merge.spec.js
+++ b/test/core/merge.spec.js
@@ -1,45 +1,131 @@
 const expect = require('chai').expect;
 const { describe } = require('node-tdd');
+const clonedeep = require('lodash.clonedeep');
 const Merge = require('../../src/core/merge');
+const genData = require('./gen-data');
 
 describe('Testing Merge', () => {
-  it('Testing SO question: https://stackoverflow.com/questions/65822248', ({ fixture }) => {
-    const json1 = fixture('json1');
-    const json2 = fixture('json2');
-    const merge = Merge({
-      '[*]': 'id',
-      '[*].addresses[*]': 'type'
-    });
-    expect(merge(json1, json2)).to.deep.equal(fixture('result'));
-  });
+  describe('Default Merge', () => {
+    let merge;
+    before(() => {
+      merge = Merge();
+    });
 
-  it('Testing string merge', () => {
-    const d1 = 'A';
-    const d2 = 'B';
-    expect(Merge()(d1, d2)).to.deep.equal('B');
-  });
+    it('Batch test', ({ fixture }) => {
+      const mergeRec = fixture('merge-rec');
+      for (let x = 0; x < 10000; x += 1) {
+        const tree = genData();
+        const subtree = genData();
+        const treeX = clonedeep(tree);
+        const subtreeX = clonedeep(subtree);
+        const r1 = merge(tree, subtree);
+        expect(treeX).to.deep.equal(tree);
+        expect(subtreeX).to.deep.equal(subtree);
+        const r2 = mergeRec(tree, subtree);
+        expect(treeX).to.deep.equal(tree);
+        expect(subtreeX).to.deep.equal(subtree);
+        expect(r1).to.deep.equal(r2);
+      }
+    });
 
-  it('Testing array concat', () => {
-    const d1 = [{ a: 1 }];
-    const d2 = [{ a: 2 }];
-    expect(Merge()(d1, d2)).to.deep.equal([{ a: 1 }, { a: 2 }]);
-  });
+    it('Nested empty array concat', () => {
+      expect(merge([[]], [[]])).to.deep.equal([[], []]);
+    });
+
+    it('Nested arrays in object concat', () => {
+      expect(merge(
+        { B: [{}, [2]] },
+        { B: [1] }
+      )).to.deep.equal({ B: [{}, [2], 1] });
+    });
+
+    it('Nested array concat objects', () => {
+      expect(merge([{ A: [] }], [0])).to.deep.equal([{ A: [] }, 0]);
+    });
+
+    it('Nested array overwrite with object', () => {
+      expect(merge({ B: [{}, {}] }, { B: {} })).to.deep.equal({ B: {} });
+    });
+
+    it('Nested nested array concat', () => {
+      expect(merge({ B: [[2]] }, { B: [[2]] })).to.deep.equal({ B: [[2], [2]] });
+    });
+
+    it('Object overwrite array', () => {
+      expect(merge([1], { a: 1 })).to.deep.equal({ a: 1 });
+    });
 
-  it('Testing array merge', () => {
-    const d1 = [{ a: 1, b: 1, c: 3 }];
-    const d2 = [{ a: 1, b: 2, d: 4 }];
-    expect(Merge({ '[*]': 'a' })(d1, d2)).to.deep.equal([{ ...d1[0], ...d2[0] }]);
+    it('Array overwrite object', () => {
+      expect(merge({ a: 1 }, [1])).to.deep.equal([1]);
+    });
+
+    it('Testing string merge', () => {
+      const d1 = 'A';
+      const d2 = 'B';
+      expect(merge(d1, d2)).to.deep.equal('B');
+    });
+
+    it('Testing array concat', () => {
+      const d1 = [{ a: 1 }];
+      const d2 = [{ a: 2 }];
+      expect(merge(d1, d2)).to.deep.equal([{ a: 1 }, { a: 2 }]);
+    });
   });
 
-  it('Testing merge by sum', () => {
-    const d1 = [[1, 2, 3], [2, 4], [1, 2]];
-    const d2 = [[3, 3], [1, 5], [3, 2]];
-    expect(Merge({
-      '[*]': (o) => o.reduce((a, b) => a + b, 0)
-    })(d1, d2)).to.deep.equal([
-      [1, 2, 3, 2, 4, 3, 3, 1, 5],
-      [1, 2],
-      [3, 2]
-    ]);
+  describe('Custom Merge', () => {
+    it('Batch test', () => {
+      const merge = Merge({ '**': 'A' });
+      for (let x = 0; x < 10000; x += 1) {
+        const tree = genData();
+        const subtree = genData();
+        const treeX = clonedeep(tree);
+        const subtreeX = clonedeep(subtree);
+        expect(() => merge(tree, subtree)).to.not.throw();
+        expect(treeX).to.deep.equal(tree);
+        expect(subtreeX).to.deep.equal(subtree);
+      }
+    });
+
+    it('Testing SO question: https://stackoverflow.com/questions/65822248', ({ fixture }) => {
+      const json1 = fixture('json1');
+      const json2 = fixture('json2');
+      const merge = Merge({
+        '[*]': 'id',
+        '[*].addresses[*]': 'type'
+      });
+      expect(merge(json1, json2)).to.deep.equal(fixture('result'));
+    });
+
+    it('Testing array merge', () => {
+      const d1 = [{ a: 1, b: 1, c: 3 }];
+      const d2 = [{ a: 1, b: 2, d: 4 }];
+      expect(Merge({ '[*]': 'a' })(d1, d2)).to.deep.equal([{ ...d1[0], ...d2[0] }]);
+    });
+
+    it('Testing merge by sum', () => {
+      const d1 = [[1, 2, 3], [2, 4], [1, 2]];
+      const d2 = [[3, 3], [1, 5], [3, 2]];
+      expect(Merge({
+        '[*]': (o) => o.reduce((a, b) => a + b, 0)
+      })(d1, d2)).to.deep.equal([
+        [1, 2, 3, 2, 4, 3, 3, 1, 5],
+        [1, 2],
+        [3, 2]
+      ]);
+    });
+
+    it('Testing incompatible', () => {
+      const d1 = [{ a: 1 }];
+      const d2 = [undefined];
+      const d3 = ['A'];
+      expect(Merge({ '[*]': 'a' })(d1, d2, d3)).to.deep.equal([{ a: 1 }, undefined]);
+    });
+
+    it('Testing overwrite', () => {
+      const d1 = { a: [{ a: 1 }] };
+      const d2 = { a: [{ a: 2 }] };
+      const d3 = { a: null };
+      expect(Merge({ '[*]': 'a' })(d1, d2, d3)).to.deep.equal({ a: null });
+    });
   });
 });
diff --git a/test/core/merge.spec.js__fixtures/merge-rec.js b/test/core/merge.spec.js__fixtures/merge-rec.js
new file mode 100644
index 0000000..69a9d2f
--- /dev/null
+++ b/test/core/merge.spec.js__fixtures/merge-rec.js
@@ -0,0 +1,28 @@
+const mergeRec = (tree, subtree) => {
+  if (!(subtree instanceof Object)) {
+    return subtree;
+  }
+  const treeIsArray = Array.isArray(tree);
+  const subtreeIsArray = Array.isArray(subtree);
+  if (treeIsArray !== subtreeIsArray) {
+    return subtreeIsArray ? [...subtree] : { ...subtree };
+  }
+  if (tree instanceof Object && subtree instanceof Object) {
+    const e1 = Object.entries(tree);
+    const e2 = Object.entries(subtree);
+    const r = Array.isArray(subtree) ? [] : {};
+    [e1, e2].forEach((e) => {
+      e.forEach(([k, v]) => {
+        if (Array.isArray(r)) {
+          r.push(v);
+        } else {
+          r[k] = k in r ? mergeRec(r[k], v) : v;
+        }
+      });
+    });
+    return r;
+  }
+  return subtree;
+};
+
+module.exports = mergeRec;