diff --git a/src/HDF5Plugin-Zstandard/zstd_h5plugin.c b/src/HDF5Plugin-Zstandard/zstd_h5plugin.c
index 5a3dfd38..a527f1ff 100644
--- a/src/HDF5Plugin-Zstandard/zstd_h5plugin.c
+++ b/src/HDF5Plugin-Zstandard/zstd_h5plugin.c
@@ -35,8 +35,8 @@ DLL_EXPORT size_t zstd_filter(unsigned int flags, size_t cd_nelmts,
 			aggression = (int)cd_values[0];
 		else
 			aggression = ZSTD_CLEVEL_DEFAULT;
-		if (aggression < 1 /*ZSTD_minCLevel()*/)
-			aggression = 1 /*ZSTD_minCLevel()*/;
+		if (aggression < ZSTD_minCLevel())
+			aggression = ZSTD_minCLevel();
 		else if (aggression > ZSTD_maxCLevel())
 			aggression = ZSTD_maxCLevel();
 
diff --git a/src/hdf5plugin/__init__.py b/src/hdf5plugin/__init__.py
index e6cc8e5a..59f4493c 100644
--- a/src/hdf5plugin/__init__.py
+++ b/src/hdf5plugin/__init__.py
@@ -391,7 +391,7 @@ class Zstd(_FilterRefClass):
             **hdf5plugin.Zstd())
         f.close()
 
-    :param int clevel: Compression level from 1 (lowest compression) to 22 (maximum compression).
+    :param int clevel: Compression level from -131072 (lowest compression) to 22 (maximum compression).
         Ultra compression extends from 20 through 22. Default: 3.
 
     .. code-block:: python
@@ -405,11 +405,13 @@ class Zstd(_FilterRefClass):
     """
     filter_id = ZSTD_ID
 
-    def __init__(self, clevel=3):
-        assert 1 <= clevel <= 22
-        self.filter_options = (clevel,)
-
+    def __init__(self, clevel=None):
+        if clevel is not None:
+             # filter_options are passed as uints: conversion needed to pass negative clevel
+            clevel = _ctypes.c_uint(clevel).value
+            self.filter_options = (clevel,)
 
+            
 def _init_filters():
     """Initialise and register HDF5 filters with h5py