diff --git a/vllm/v1/core/sched/scheduler.py b/vllm/v1/core/sched/scheduler.py
index d39aea1f2d11..430085d9c978 100644
--- a/vllm/v1/core/sched/scheduler.py
+++ b/vllm/v1/core/sched/scheduler.py
@@ -437,14 +437,24 @@ class Scheduler(SchedulerInterface):
                             # The request cannot be scheduled.
                             break
 
+                # Handles an edge case when P/D Disaggregation
+                # is used with Spec Decoding where an
+                # extra block gets allocated which
+                # creates a mismatch between the number
+                # of local and remote blocks.
+                effective_lookahead_tokens = (0 if request.num_computed_tokens
+                                              == 0 else
+                                              self.num_lookahead_tokens)
+
                 new_blocks = self.kv_cache_manager.allocate_slots(
                     request,
                     num_new_tokens + num_external_computed_tokens,
                     num_new_local_computed_tokens,
                     new_computed_blocks,
-                    num_lookahead_tokens=self.num_lookahead_tokens,
+                    num_lookahead_tokens=effective_lookahead_tokens,
                     delay_cache_blocks=load_kv_async,
                 )
+
                 if new_blocks is None:
                     # The request cannot be scheduled.
                     break