diff --git a/.changeset/nine-eagles-fetch.md b/.changeset/nine-eagles-fetch.md
new file mode 100644
index 0000000000..e4102c04fb
--- /dev/null
+++ b/.changeset/nine-eagles-fetch.md
@@ -0,0 +1,5 @@
+---
+'@core/sync-service': patch
+---
+
+Fix Materializer crash that was happening when processing changes with PK updates.
diff --git a/packages/sync-service/lib/electric/shapes/consumer/materializer.ex b/packages/sync-service/lib/electric/shapes/consumer/materializer.ex
index c668f17745..df6baf2f5e 100644
--- a/packages/sync-service/lib/electric/shapes/consumer/materializer.ex
+++ b/packages/sync-service/lib/electric/shapes/consumer/materializer.ex
@@ -377,11 +377,15 @@ defmodule Electric.Shapes.Consumer.Materializer do
 
           %Changes.UpdatedRecord{
             key: key,
+            old_key: old_key,
             record: record,
             move_tags: move_tags,
             removed_move_tags: removed_move_tags
           },
           {{index, tag_indices}, counts_and_events} ->
+            # When the primary key doesn't change, old_key may be nil; default to key
+            old_key = old_key || key
+
             # TODO: this is written as if it supports multiple selected columns, but it doesn't for now
             columns_present = Enum.any?(state.columns, &is_map_key(record, &1))
             has_tag_updates = removed_move_tags != []
@@ -389,12 +393,12 @@ defmodule Electric.Shapes.Consumer.Materializer do
             if columns_present or has_tag_updates do
               tag_indices =
                 tag_indices
-                |> remove_row_from_tag_indices(key, removed_move_tags)
+                |> remove_row_from_tag_indices(old_key, removed_move_tags)
                 |> add_row_to_tag_indices(key, move_tags)
 
               if columns_present do
                 {value, original_string} = cast!(record, state)
-                old_value = Map.fetch!(index, key)
+                {old_value, index} = Map.pop!(index, old_key)
                 index = Map.put(index, key, value)
 
                 # Skip decrement/increment dance if value hasn't changed to avoid
diff --git a/packages/sync-service/test/electric/shapes/consumer/materializer_test.exs b/packages/sync-service/test/electric/shapes/consumer/materializer_test.exs
index 5f4df78afc..213f57d90a 100644
--- a/packages/sync-service/test/electric/shapes/consumer/materializer_test.exs
+++ b/packages/sync-service/test/electric/shapes/consumer/materializer_test.exs
@@ -350,6 +350,60 @@ defmodule Electric.Shapes.Consumer.MaterializerTest do
       end)
     end
 
+    @tag snapshot_data: {
+           [%Changes.NewRecord{record: %{"id" => "1", "value" => "10"}}],
+           [pk_cols: ["id"]]
+         }
+    test "update that changes the primary key is handled correctly", ctx do
+      ctx = with_materializer(ctx)
+
+      assert Materializer.get_link_values(ctx) == MapSet.new([10])
+
+      # Update where the PK changes from "1" to "2"
+      Materializer.new_changes(
+        ctx,
+        [
+          %Changes.UpdatedRecord{
+            record: %{"id" => "2", "value" => "20"},
+            old_record: %{"id" => "1", "value" => "10"}
+          }
+        ]
+        |> prep_changes()
+      )
+
+      assert Materializer.get_link_values(ctx) == MapSet.new([20])
+
+      assert_receive {:materializer_changes, _, %{move_out: [{10, "10"}], move_in: [{20, "20"}]}}
+    end
+
+    @tag snapshot_data: {
+           [%Changes.NewRecord{record: %{"id" => "1", "value" => "10"}}],
+           [pk_cols: ["id"]]
+         }
+    test "update that changes the primary key but keeps the same value", ctx do
+      ctx = with_materializer(ctx)
+
+      assert Materializer.get_link_values(ctx) == MapSet.new([10])
+
+      # Update where the PK changes but tracked value stays the same
+      Materializer.new_changes(
+        ctx,
+        [
+          %Changes.UpdatedRecord{
+            record: %{"id" => "2", "value" => "10"},
+            old_record: %{"id" => "1", "value" => "10"}
+          }
+        ]
+        |> prep_changes()
+      )
+
+      # Value should still be present
+      assert Materializer.get_link_values(ctx) == MapSet.new([10])
+
+      # No events since the tracked value didn't change
+      refute_received {:materializer_changes, _, _}
+    end
+
     test "events are accumulated across uncommitted fragments", ctx do
       ctx = with_materializer(ctx)