diff --git a/nibabel/casting.py b/nibabel/casting.py
index 410986050..229013512 100644
--- a/nibabel/casting.py
+++ b/nibabel/casting.py
@@ -27,7 +27,7 @@ class CastingError(Exception):
 sctypes = {
     'int': [np.int8, np.int16, np.int32, np.int64],
     'uint': [np.uint8, np.uint16, np.uint32, np.uint64],
-    'float': [np.float16, np.float32, np.float64, np.float128],
+    'float': [np.float16, np.float32, np.float64, np.longdouble],
     'complex': [np.complex64, np.complex128, np.complex256],
     'others': [bool, object, bytes, str, np.void],
 }