diff --git a/pymc/backends/__init__.py b/pymc/backends/__init__.py
index 9347d2ae3a1..1be86110a88 100644
--- a/pymc/backends/__init__.py
+++ b/pymc/backends/__init__.py
@@ -73,7 +73,7 @@
 
 HAS_MCB = False
 try:
-    from mcbackend import Backend
+    from mcbackend import Backend, NumPyBackend
 
     from pymc.backends.mcbackend import init_chain_adapters
 
@@ -119,6 +119,8 @@ def init_traces(
     model: Model,
 ) -> Sequence[IBaseTrace]:
     """Initializes a trace recorder for each chain."""
+    if HAS_MCB and backend is None:
+        backend = NumPyBackend(preallocate=expected_length)
     if HAS_MCB and isinstance(backend, Backend):
         return init_chain_adapters(
             backend=backend,