diff --git a/store/postgres/src/deployment_store.rs b/store/postgres/src/deployment_store.rs
index d764d451ad6..9162f5ffc9e 100644
--- a/store/postgres/src/deployment_store.rs
+++ b/store/postgres/src/deployment_store.rs
@@ -993,6 +993,20 @@ impl DeploymentStore {
             // Unfortunately the machinery needed to do this is not yet in place.
             // The best we can do right now is just to make sure that the block number
             // is high enough.
+            let state = deployment::state(&mut conn, site.as_ref()).await?;
+            if block.number < state.earliest_block_number {
+                // Pruning removed the causality-region digests this block's POI is
+                // computed from. Querying anyway returns a well-formed but wrong POI
+                // that nobody can tell apart from a correct one, which is worse than
+                // failing: an allocation closing against a pruned epoch block would
+                // submit it and could be disputed.
+                return Err(StoreError::Unknown(anyhow!(
+                    "block {} has been pruned for deployment {}; the earliest available block is {}",
+                    block.number,
+                    site.deployment,
+                    state.earliest_block_number
+                )));
+            }
             if latest_block_ptr.number < block.number {
                 // If a subgraph has failed deterministically then any blocks past head
                 // should return the same POI
diff --git a/store/test-store/tests/postgres.rs b/store/test-store/tests/postgres.rs
index 1d5a16ba851..d1be7b8fa53 100644
--- a/store/test-store/tests/postgres.rs
+++ b/store/test-store/tests/postgres.rs
@@ -2,6 +2,7 @@ pub mod postgres {
     pub mod aggregation;
     pub mod chain_head;
     pub mod graft;
+    pub mod pruned_poi;
     pub mod relational;
     pub mod relational_bytes;
     pub mod store;
diff --git a/store/test-store/tests/postgres/pruned_poi.rs b/store/test-store/tests/postgres/pruned_poi.rs
new file mode 100644
index 00000000000..aebaa5f92e5
--- /dev/null
+++ b/store/test-store/tests/postgres/pruned_poi.rs
@@ -0,0 +1,75 @@
+// A proof of indexing for a block that pruning has already removed cannot be computed:
+// the causality-region digests it is hashed from are gone. Returning a well-formed but
+// wrong POI is worse than failing, because nothing downstream can tell the difference.
+
+use graph::prelude::{DeploymentHash, SubgraphStore as _};
+use test_store::*;
+
+const SUBGRAPH_GQL: &str = "
+    type User @entity {
+        id: ID!,
+        name: String
+    }
+";
+
+/// The block pointer the POS tests use: a deterministic hash per number.
+fn ptr(n: i32) -> graph::prelude::BlockPtr {
+    let hash = graph::prelude::alloy::primitives::B256::from(
+        graph::prelude::alloy::primitives::U256::from(n as u64),
+    );
+    graph::prelude::BlockPtr::new(hash.into(), n.into())
+}
+
+/// Put the subgraph at `head` and move its history window forward to `earliest`,
+/// which is what the pruner leaves behind once it has passed the blocks below it.
+async fn set_head_and_prune(deployment: &DeploymentHash, head: i32, earliest: i32) {
+    let mut conn = PRIMARY_POOL.get().await.unwrap();
+    let hash = deployment.as_str().to_string();
+    let hash_bytes = format!("\\x{}", "11".repeat(32));
+    let stmt = format!(
+        "UPDATE subgraphs.deployment SET earliest_block_number = {earliest} WHERE subgraph = '{hash}';\
+         UPDATE subgraphs.head SET block_number = {head}, block_hash = '{hash_bytes}' WHERE id = \
+           (SELECT id FROM subgraphs.deployment WHERE subgraph = '{hash}');"
+    );
+    diesel_async::SimpleAsyncConnection::batch_execute(&mut conn, &stmt)
+        .await
+        .unwrap();
+}
+
+#[test]
+fn proof_of_indexing_refuses_a_pruned_block() {
+    run_test_sequentially(|store| async move {
+        let id = DeploymentHash::new("prunedPoi").unwrap();
+        create_test_subgraph(&id, SUBGRAPH_GQL).await;
+
+        // The subgraph is at block 200; everything below 100 has been pruned away.
+        set_head_and_prune(&id, 200, 100).await;
+
+        let sg_store = store.subgraph_store();
+        let indexer = None;
+
+        // A retained block is unaffected.
+        let retained = sg_store
+            .get_proof_of_indexing(&id, &indexer, ptr(120))
+            .await;
+        assert!(
+            retained.is_ok(),
+            "a retained block must still produce a POI: {retained:?}"
+        );
+
+        // A pruned block must be an error, not a silently wrong hash.
+        let pruned = sg_store
+            .get_proof_of_indexing(&id, &indexer, ptr(50))
+            .await;
+        match pruned {
+            Err(e) => {
+                let msg = format!("{e}");
+                assert!(
+                    msg.contains("pruned") || msg.contains("earliest"),
+                    "the error should say the block was pruned, got: {msg}"
+                );
+            }
+            Ok(v) => panic!("a pruned block returned {v:?} instead of an error"),
+        }
+    })
+}