diff --git a/index.js b/index.js
index 42e1ddc..0ddfd68 100644
--- a/index.js
+++ b/index.js
@@ -17,8 +17,9 @@ export function flatten (target, opts) {
   const transformKey = opts.transformKey || keyIdentity
   const output = {}
 
-  function step (object, prev, currentDepth) {
+  function step (object, prev, currentDepth, referenceMap = new Map) {
     currentDepth = currentDepth || 1
+    referenceMap.set(object, prev || "object-root");
     Object.keys(object).forEach(function (key) {
       const value = object[key]
       const isarray = opts.safe && Array.isArray(value)
@@ -35,11 +36,16 @@ export function flatten (target, opts) {
 
       if (!isarray && !isbuffer && isobject && Object.keys(value).length &&
         (!opts.maxDepth || currentDepth < maxDepth)) {
-        return step(value, newKey, currentDepth + 1)
+
+        if(referenceMap.has(value)) 
+          throw new Error("Circular structure on key '" + key + "' (to '" + referenceMap.get(value) + "')");
+
+          return step(value, newKey, currentDepth + 1, referenceMap)
       }
 
       output[newKey] = value
-    })
+    });
+    referenceMap.delete(object);
   }
 
   step(target)
diff --git a/test/test.js b/test/test.js
index ef01c9b..2093df5 100644
--- a/test/test.js
+++ b/test/test.js
@@ -206,6 +206,39 @@ describe('Flatten', function () {
       'hello.0500': 'darkness my old friend'
     })
   })
+
+  test('Cyclic object - With depth cycle reference', function() {
+    let objA = { h:"hello" }
+    objA.propA = { A_h:"hello" }
+    objA.propA._propA = { _A_h:"hello" }
+    objA.propA._propA.cycle = objA.propA;
+    
+    try { 
+      flatten(objA);
+    } catch(e) {
+      if(e instanceof RangeError)
+        throw e;
+    }
+
+  })
+
+  test('Cyclic object - Inner reference without cycle', function() {
+    let objA = { h:"hello1" }
+    objA.propA = { A_h:"hello2" }
+    objA.propB = { B_h:"hello3" }
+    objA.propA._propA = { _A_h:"hello4" }
+    objA.propA._propA.no_cycle = objA.propB;
+    
+    assert.deepStrictEqual(flatten(objA),{
+      "h":"hello1",
+      "propA.A_h":"hello2",
+      "propA._propA._A_h":"hello4",
+      "propA._propA.no_cycle.B_h":"hello3",
+      "propB.B_h":"hello3",
+    })
+  })
+
+  
 })
 
 describe('Unflatten', function () {