diff --git a/PyPDF2/merger.py b/PyPDF2/merger.py
index 27702ad..c3373e4 100644
--- a/PyPDF2/merger.py
+++ b/PyPDF2/merger.py
@@ -113,7 +113,7 @@ class PdfFileMerger(object):
         if isString(fileobj):
             fileobj = file(fileobj, 'rb')
             my_file = True
-        elif isinstance(fileobj, file):
+        elif hasattr(fileobj, "seek") and hasattr(fileobj, "read"):
             fileobj.seek(0)
             filecontent = fileobj.read()
             fileobj = StreamIO(filecontent)