diff --git a/python/triton/backends/driver.py b/python/triton/backends/driver.py
index 44a4253efa64..0ec215ea3a89 100644
--- a/python/triton/backends/driver.py
+++ b/python/triton/backends/driver.py
@@ -146,6 +146,14 @@ def get_benchmarker(self) -> Benchmarker:
         """
         raise NotImplementedError
 
+    def allocate_default_global_scratch(self, size: int, alignment: int, stream):
+        """
+        Allocate global scratch when no explicit allocator was installed via set_allocator().
+        Kernels on Blackwell (SM 12.0+) may require global scratch memory for cooperative
+        operations. This fallback prevents RuntimeError when no allocator is configured.
+        """
+        raise NotImplementedError
+
     def allocate_default_profile_scratch(self, size: int, alignment: int, stream):
         """
         Allocate profile scratch when no explicit profile allocator override was installed.
@@ -174,6 +182,18 @@ def __init__(self):
     def assemble_tensormap_to_arg(self, tensormaps_info, args):
         return args
 
+    def allocate_default_global_scratch(self, size: int, alignment: int, stream):
+        import torch
+        device = self.get_active_torch_device()
+        device_interface = self.get_device_interface()
+        if stream is None:
+            return torch.empty(size, dtype=torch.uint8, device=device).data_ptr()
+        launch_stream = device_interface.ExternalStream(stream, device=device)
+        with device_interface.stream(launch_stream):
+            scratch = torch.empty(size, dtype=torch.uint8, device=device)
+        scratch.record_stream(launch_stream)
+        return scratch.data_ptr()
+
     def allocate_default_profile_scratch(self, size: int, alignment: int, stream):
         import torch
         device = self.get_active_torch_device()
diff --git a/third_party/amd/backend/driver.py b/third_party/amd/backend/driver.py
index bc7cee8972c9..ebefa34d3520 100644
--- a/third_party/amd/backend/driver.py
+++ b/third_party/amd/backend/driver.py
@@ -349,6 +349,8 @@ def allocate_scratch(size, align, allocator):
                 grid_size = gridX * gridY * gridZ
                 alloc_size = grid_size * size
                 alloc_fn = allocator.get()
+                if isinstance(alloc_fn, _allocation.NullAllocator):
+                    return active_driver.allocate_default_global_scratch(alloc_size, align, stream)
                 return alloc_fn(alloc_size, align, stream)
             return None
 
diff --git a/third_party/nvidia/backend/driver.py b/third_party/nvidia/backend/driver.py
index 9ea4894660e9..0207779a3dcb 100644
--- a/third_party/nvidia/backend/driver.py
+++ b/third_party/nvidia/backend/driver.py
@@ -303,6 +303,8 @@ def allocate_scratch(size, align, allocator):
                 grid_size = gridX * gridY * gridZ
                 alloc_size = grid_size * self.num_ctas * size
                 alloc_fn = allocator.get()
+                if isinstance(alloc_fn, _allocation.NullAllocator):
+                    return active_driver.allocate_default_global_scratch(alloc_size, align, stream)
                 return alloc_fn(alloc_size, align, stream)
             return None