diff --git a/gnupg/gnupg.py b/gnupg/gnupg.py
index 4ec7879..b23cecb 100644
--- a/gnupg/gnupg.py
+++ b/gnupg/gnupg.py
@@ -186,7 +186,7 @@ class GPG(GPGBase):
         else:
             log.warn("No 'default_key' given! Using first key on secring.")
 
-        if isinstance(data, file):
+        if hasattr(data, 'read'):
             result = self._sign_file(data, **kwargs)
         elif not _is_stream(data):
             stream = _make_binary_stream(data, self._encoding)