diff --git a/gunicorn/http/_sendfile.py b/gunicorn/http/_sendfile.py
index b31009ed..a822a851 100644
--- a/gunicorn/http/_sendfile.py
+++ b/gunicorn/http/_sendfile.py
@@ -38,7 +38,7 @@ def sendfile(fdout, fdin, offset, nbytes):
         
         if result == -1:
             e = ctypes.get_errno()
-            if e == errno.EAGAIN and _nbytes.value:
+            if e == errno.EAGAIN and _nbytes.value is not None:
                 return _nbytes.value
             raise OSError(e, os.strerror(e))
         return _nbytes.value
@@ -50,7 +50,7 @@ def sendfile(fdout, fdin, offset, nbytes):
         result = _sendfile(fdin, fdout, offset, nbytes, None, _sbytes, 0)
         if result == -1:
             e = ctypes.get_errno()
-            if e == errno.EAGAIN and _sbytes.value:
+            if e == errno.EAGAIN and _sbytes.value is not None:
                 return _sbytes.value
             raise OSError(e, os.strerror(e))
         return _sbytes.value