diff --git a/src/operator/mergeMap.ts b/src/operator/mergeMap.ts
index b5076f0ef4..71e1b98209 100644
--- a/src/operator/mergeMap.ts
+++ b/src/operator/mergeMap.ts
@@ -2,8 +2,6 @@ import {Observable} from '../Observable';
 import {Operator} from '../Operator';
 import {Subscriber} from '../Subscriber';
 import {Subscription} from '../Subscription';
-import {tryCatch} from '../util/tryCatch';
-import {errorObject} from '../util/errorObject';
 import {subscribeToResult} from '../util/subscribeToResult';
 import {OuterSubscriber} from '../OuterSubscriber';
 
@@ -39,7 +37,7 @@ export class MergeMapOperator<T, R, R2> implements Operator<T, R> {
 
 export class MergeMapSubscriber<T, R, R2> extends OuterSubscriber<T, R> {
   private hasCompleted: boolean = false;
-  private buffer: T[] = [];
+  private buffer: Observable<any>[] = [];
   private active: number = 0;
   protected index: number = 0;
 
@@ -50,23 +48,28 @@ export class MergeMapSubscriber<T, R, R2> extends OuterSubscriber<T, R> {
     super(destination);
   }
 
-  protected _next(value: T): void {
+  protected _next(value: any): void {
     if (this.active < this.concurrent) {
-      const index = this.index++;
-      const ish = tryCatch(this.project)(value, index);
-      const destination = this.destination;
-      if (ish === errorObject) {
-        destination.error(errorObject.e);
-      } else {
-        this.active++;
-        this._innerSub(ish, value, index);
-      }
+      this._tryNext(value);
     } else {
       this.buffer.push(value);
     }
   }
 
-  private _innerSub(ish: Observable<R>, value: T, index: number): void {
+  protected _tryNext(value: any) {
+    let result: any;
+    const index = this.index++;
+    try {
+      result = this.project(value, index);
+    } catch (err) {
+      this.destination.error(err);
+      return;
+    }
+    this.active++;
+    this._innerSub(result, value, index);
+  }
+
+  private _innerSub(ish: any, value: T, index: number): void {
     this.add(subscribeToResult<T, R>(this, ish, value, index));
   }
 
@@ -78,17 +81,22 @@ export class MergeMapSubscriber<T, R, R2> extends OuterSubscriber<T, R> {
   }
 
   notifyNext(outerValue: T, innerValue: R, outerIndex: number, innerIndex: number): void {
-    const { destination, resultSelector } = this;
-    if (resultSelector) {
-      const result = tryCatch(resultSelector)(outerValue, innerValue, outerIndex, innerIndex);
-      if (result === errorObject) {
-        destination.error(errorObject.e);
-      } else {
-        destination.next(result);
-      }
+    if (this.resultSelector) {
+      this._notifyResultSelector(outerValue, innerValue, outerIndex, innerIndex);
     } else {
-      destination.next(innerValue);
+      this.destination.next(innerValue);
+    }
+  }
+
+  _notifyResultSelector(outerValue: T, innerValue: R, outerIndex: number, innerIndex: number) {
+    let result: any;
+    try {
+      result = this.resultSelector(outerValue, innerValue, outerIndex, innerIndex);
+    } catch (err) {
+      this.destination.error(err);
+      return;
     }
+    this.destination.next(result);
   }
 
   notifyComplete(innerSub: Subscription): void {