diff --git a/pgvector/utils/sparsevec.py b/pgvector/utils/sparsevec.py
index c21e0d3..a451fc6 100644
--- a/pgvector/utils/sparsevec.py
+++ b/pgvector/utils/sparsevec.py
@@ -1,18 +1,23 @@
 import numpy as np
 from struct import pack, unpack_from
 
+NO_DEFAULT = object()
+
 
 class SparseVector:
-    def __init__(self, value, dimensions=None, /):
+    def __init__(self, value, dimensions=NO_DEFAULT, /):
         if value.__class__.__module__ == 'scipy.sparse._arrays':
-            if dimensions is not None:
+            if dimensions is not NO_DEFAULT:
                 raise ValueError('dimensions not allowed')
 
             self._from_sparse(value)
         elif isinstance(value, dict):
+            if dimensions is NO_DEFAULT:
+                raise ValueError('dimensions required')
+
             self._from_dict(value, dimensions)
         else:
-            if dimensions is not None:
+            if dimensions is not NO_DEFAULT:
                 raise ValueError('dimensions not allowed')
 
             self._from_dense(value)
@@ -56,9 +61,6 @@ def to_binary(self):
         return pack(f'>iii{nnz}i{nnz}f', self._dim, nnz, 0, *self._indices, *self._values)
 
     def _from_dict(self, d, dim):
-        if dim is None:
-            raise ValueError('dimensions required')
-
         elements = [(i, v) for i, v in d.items() if v != 0]
         elements.sort()