diff --git a/PSOperations/GroupOperation.swift b/PSOperations/GroupOperation.swift
index bc38362..e43aa28 100644
--- a/PSOperations/GroupOperation.swift
+++ b/PSOperations/GroupOperation.swift
@@ -25,6 +25,7 @@ open class GroupOperation: Operation {
     fileprivate let internalQueue = OperationQueue()
     fileprivate let startingOperation = Foundation.BlockOperation(block: {})
     fileprivate let finishingOperation = Foundation.BlockOperation(block: {})
+    fileprivate let errorsLock = NSLock()
 
     fileprivate var aggregatedErrors = [NSError]()
     
@@ -65,7 +66,9 @@ open class GroupOperation: Operation {
         of errors reported to observers and to the `finished(_:)` method.
     */
     public final func aggregateError(_ error: NSError) {
+        errorsLock.lock()
         aggregatedErrors.append(error)
+        errorsLock.unlock()
     }
     
     open func operationDidFinish(_ operation: Foundation.Operation, withErrors errors: [NSError]) {
@@ -100,7 +103,9 @@ extension GroupOperation: OperationQueueDelegate {
     }
     
     final public func operationQueue(_ operationQueue: OperationQueue, operationDidFinish operation: Foundation.Operation, withErrors errors: [NSError]) {
+        errorsLock.lock()
         aggregatedErrors.append(contentsOf: errors)
+        errorsLock.unlock()
         
         if operation === finishingOperation {
             internalQueue.isSuspended = true