diff --git a/numcodecs/tests/test_zarr3.py b/numcodecs/tests/test_zarr3.py
index a40b658f..0d8ecc74 100644
--- a/numcodecs/tests/test_zarr3.py
+++ b/numcodecs/tests/test_zarr3.py
@@ -91,7 +91,7 @@ def test_generic_compressor(
         (numcodecs.zarr3.Delta, {"dtype": "float32"}),
         (numcodecs.zarr3.FixedScaleOffset, {"offset": 0, "scale": 25.5}),
         (numcodecs.zarr3.FixedScaleOffset, {"offset": 0, "scale": 51, "astype": "uint16"}),
-        (numcodecs.zarr3.AsType, {"encode_dtype": "float32", "decode_dtype": "float64"}),
+        (numcodecs.zarr3.AsType, {"encode_dtype": "float32", "decode_dtype": "float32"}),
     ],
     ids=[
         "delta",
diff --git a/numcodecs/zarr3.py b/numcodecs/zarr3.py
index 78293514..f1743ffb 100644
--- a/numcodecs/zarr3.py
+++ b/numcodecs/zarr3.py
@@ -271,7 +271,7 @@ def __init__(self, **codec_config: JSON) -> None:
         super().__init__(**codec_config)
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Shuffle:
-        if array_spec.dtype.itemsize != self.codec_config.get("elementsize"):
+        if self.codec_config.get("elementsize", None) is None:
             return Shuffle(**{**self.codec_config, "elementsize": array_spec.dtype.itemsize})
         return self  # pragma: no cover
 
@@ -308,7 +308,7 @@ def resolve_metadata(self, chunk_spec: ArraySpec) -> ArraySpec:
         return chunk_spec
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> FixedScaleOffset:
-        if str(array_spec.dtype) != self.codec_config.get("dtype"):
+        if self.codec_config.get("dtype", None) is None:
             return FixedScaleOffset(**{**self.codec_config, "dtype": str(array_spec.dtype)})
         return self
 
@@ -321,7 +321,7 @@ def __init__(self, **codec_config: JSON) -> None:
         super().__init__(**codec_config)
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Quantize:
-        if str(array_spec.dtype) != self.codec_config.get("dtype"):
+        if self.codec_config.get("dtype", None) is None:
             return Quantize(**{**self.codec_config, "dtype": str(array_spec.dtype)})
         return self
 
@@ -356,8 +356,7 @@ def resolve_metadata(self, chunk_spec: ArraySpec) -> ArraySpec:
         return replace(chunk_spec, dtype=np.dtype(self.codec_config["encode_dtype"]))  # type: ignore[arg-type]
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> AsType:
-        decode_dtype = self.codec_config.get("decode_dtype")
-        if str(array_spec.dtype) != decode_dtype:
+        if self.codec_config.get("decode_dtype", None) is None:
             return AsType(**{**self.codec_config, "decode_dtype": str(array_spec.dtype)})
         return self