diff --git a/sorts/merge_sort.py b/sorts/merge_sort.py
index ecbad7075..714861e72 100644
--- a/sorts/merge_sort.py
+++ b/sorts/merge_sort.py
@@ -37,7 +37,7 @@ def merge_sort(collection):
         '''
         result = []
         while left and right:
-            result.append(left.pop(0) if left[0] <= right[0] else right.pop(0))
+            result.append((left if left[0] <= right[0] else right).pop(0))
         return result + left + right
     if len(collection) <= 1:
         return collection
@@ -53,4 +53,4 @@ if __name__ == '__main__':
 
     user_input = raw_input('Enter numbers separated by a comma:\n').strip()
     unsorted = [int(item) for item in user_input.split(',')]
-    print(*merge_sort(unsorted), sep=',')
\ No newline at end of file
+    print(*merge_sort(unsorted), sep=',')