diff --git a/src/commands.zig b/src/commands.zig
index 2cfdda0..78b74d5 100644
--- a/src/commands.zig
+++ b/src/commands.zig
@@ -2232,7 +2232,11 @@ fn cmd_aggregate(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
             proj_pairs = doc_arg(stage[0].value);
         } else if (std.mem.eql(u8, stage_name, "$group")) {
             const gp = doc_arg(stage[0].value) orelse return bad_value(reply, "$group requires a document");
-            const grouped_opt = try run_group(ctx, reply, coll, gp, offs.items[start..end]);
+            const src: Stream = if (in_trees)
+                .{ .docs = trees.items[start..end] }
+            else
+                .{ .offsets = offs.items[start..end] };
+            const grouped_opt = try run_group(ctx, reply, coll, gp, src);
             var grouped = grouped_opt orelse return;
             // Group results replace the stream: later stages see groups.
             offs.deinit(ctx.gpa);
@@ -2346,6 +2350,60 @@ fn count_only_pipeline(reply: *wire.Reply, stages: []const bson.Value) !?CountSh
     return .{ .filter = filter, .id_value = id_value, .accs = accs.items };
 }
 
+/// Where a pipeline stage reads its input.
+///
+/// A pipeline starts as slab offsets -- matched and reordered in place, never
+/// materialized -- and flips to generated documents the moment a stage
+/// produces something that is not in the slab. Both forms are real and a stage
+/// that reads only one of them reads the wrong list.
+///
+/// `$group` used to take `[]const u64` and be handed `offs.items[start..end]`
+/// unconditionally, with `start`/`end` set from whichever list was live. After
+/// a stage that materializes, `offs` is empty and the bounds are the tree
+/// count, so a second `$group` sliced an empty list with a non-zero end and
+/// panicked the server. Any client could send it.
+const Stream = union(enum) {
+    offsets: []const u64,
+    docs: []const *const bson.Document,
+
+    fn len(self: Stream) usize {
+        return switch (self) {
+            .offsets => |o| o.len,
+            .docs => |d| d.len,
+        };
+    }
+};
+
+/// A dotted path resolved against a document tree, the counterpart of
+/// `query_path_value_bytes` for the materialized half of a stream.
+fn path_in_pairs(pairs: []const bson.Pair, path: []const u8) ?bson.Value {
+    var it = std.mem.splitScalar(u8, path, '.');
+    var cur = bson.get_pair(pairs, it.next() orelse return null) orelse return null;
+    while (it.next()) |seg| {
+        cur = switch (cur) {
+            .doc => |p| bson.get_pair(p, seg) orelse return null,
+            else => return null,
+        };
+    }
+    return cur;
+}
+
+/// One item of a stream, resolved along `path`, whichever form the stream is
+/// in. The slab side stays byte-walked: `$group` over a million documents does
+/// not build a million trees to read one field.
+fn stream_path(
+    gpa: std.mem.Allocator,
+    coll: *const Collection,
+    src: Stream,
+    i: usize,
+    path: []const u8,
+) !?bson.Value {
+    return switch (src) {
+        .offsets => |o| try query_path_value_bytes(gpa, coll.doc_bytes(o[i]), path),
+        .docs => |d| path_in_pairs(d[i].pairs, path),
+    };
+}
+
 /// The whole vocabulary this engine can evaluate: a field path, or a constant.
 ///
 /// There is no expression evaluator (PLAN amendment A6 puts one in M2.5), and
@@ -2409,7 +2467,7 @@ fn run_group(
     reply: *wire.Reply,
     coll: *const Collection,
     group_pairs: []const bson.Pair,
-    docs: []const u64,
+    src: Stream,
 ) !?std.ArrayListUnmanaged(*const bson.Document) {
     const arena = reply.arena_alloc();
     const id_expr = bson.get_pair(group_pairs, "_id") orelse {
@@ -2485,10 +2543,10 @@ fn run_group(
     // Byte-walk materializations (nested group keys) live here.
     var walk_arena = std.heap.ArenaAllocator.init(ctx.gpa);
     defer walk_arena.deinit();
-    for (docs) |off| {
-        const doc = coll.doc_bytes(off);
+    var i: usize = 0;
+    while (i < src.len()) : (i += 1) {
         const id_value: bson.Value = switch (id_class) {
-            .path => |path| (try query_path_value_bytes(walk_arena.allocator(), doc, path)) orelse .null,
+            .path => |path| (try stream_path(walk_arena.allocator(), coll, src, i, path)) orelse .null,
             .constant => |v| v,
         };
         id_key_buf.clearRetainingCapacity();
@@ -2505,15 +2563,15 @@ fn run_group(
             @memset(sums, 0);
             gop.value_ptr.* = .{ .id_value = id_value, .sums = sums };
         }
-        for (accs.items, 0..) |acc, i| {
+        for (accs.items, 0..) |acc, a| {
             // A `$sum` over something that is not a number contributes nothing,
             // which is MongoDB's rule and not a stand-in for an unimplemented
             // one: `{$sum: "$name"}` over strings really is zero.
             const v: bson.Value = switch (acc.arg) {
-                .path => |path| (try query_path_value_bytes(walk_arena.allocator(), doc, path)) orelse .null,
+                .path => |path| (try stream_path(walk_arena.allocator(), coll, src, i, path)) orelse .null,
                 .constant => |c| c,
             };
-            gop.value_ptr.sums[i] += switch (v) {
+            gop.value_ptr.sums[a] += switch (v) {
                 .int32 => |n| @floatFromInt(n),
                 .int64 => |n| @floatFromInt(n),
                 .double => |n| n,
@@ -2534,13 +2592,13 @@ fn run_group(
         const npairs = 1 + accs.items.len;
         const pairs = try arena.alloc(bson.Pair, npairs);
         pairs[0] = .{ .key = "_id", .value = try bson.copy_value(arena, entry.value_ptr.id_value) };
-        for (accs.items, 0..) |acc, i| {
-            const sum: f64 = entry.value_ptr.sums[i];
+        for (accs.items, 0..) |acc, a| {
+            const sum: f64 = entry.value_ptr.sums[a];
             const sum_value: bson.Value = if (sum == @floor(sum) and sum <= 2_147_483_647 and sum >= -2_147_483_648)
                 .{ .int32 = @intFromFloat(sum) }
             else
                 .{ .double = sum };
-            pairs[1 + i] = .{ .key = acc.key, .value = sum_value };
+            pairs[1 + a] = .{ .key = acc.key, .value = sum_value };
         }
         const doc = try arena.create(bson.Document);
         doc.* = bson.Document{ .arena = undefined, .pairs = pairs };
@@ -3976,6 +4034,65 @@ test "aggregate $sort without a preceding $group sorts and frees correctly" {
     }
 }
 
+test "a pipeline may group what an earlier stage generated" {
+    // A remote crash, reachable by any client with a two-stage pipeline and no
+    // authentication in front of it: `$group` took `[]const u64` and was handed
+    // `offs.items[start..end]` whatever the stream was made of. After a stage
+    // that materializes -- another `$group`, and now `$project` -- `offs` is
+    // empty while the bounds count trees, so the slice ran off an empty list
+    // and panicked the server thread.
+    //
+    // "index out of bounds: index 2, len 0", measured against the binary at the
+    // previous commit before this was written.
+    //
+    // Mutation check: hand `run_group` `.{ .offsets = offs.items[start..end] }`
+    // unconditionally, and this aborts the run rather than failing it -- which
+    // is exactly why it was worth a test rather than a code read.
+    var threaded = std.Io.Threaded.init(testing.allocator, .{});
+    defer threaded.deinit();
+    const io = threaded.io();
+
+    var tdb = try TestDb.init(io);
+    defer tdb.deinit();
+    try dispatch_insert(&tdb, io, "gg", &.{
+        .{ .doc = &.{ .{ .key = "_id", .value = .{ .int32 = 1 } }, .{ .key = "k", .value = .{ .string = "a" } } } },
+        .{ .doc = &.{ .{ .key = "_id", .value = .{ .int32 = 2 } }, .{ .key = "k", .value = .{ .string = "a" } } } },
+        .{ .doc = &.{ .{ .key = "_id", .value = .{ .int32 = 3 } }, .{ .key = "k", .value = .{ .string = "b" } } } },
+    });
+
+    // Group by key, then count the groups: the second $group reads what the
+    // first one generated, which lives in no slab.
+    const by_k = bson.Value{ .doc = &.{.{ .key = "$group", .value = .{ .doc = &.{
+        .{ .key = "_id", .value = .{ .string = "$k" } },
+        .{ .key = "n", .value = .{ .doc = &.{.{ .key = "$sum", .value = .{ .int32 = 1 } }} } },
+    } } }} };
+    const count_groups = bson.Value{ .doc = &.{.{ .key = "$group", .value = .{ .doc = &.{
+        .{ .key = "_id", .value = .null },
+        .{ .key = "groups", .value = .{ .doc = &.{.{ .key = "$sum", .value = .{ .int32 = 1 } }} } },
+        // And a path that only resolves against the *generated* document,
+        // which is what makes this more than a bounds check.
+        .{ .key = "docs", .value = .{ .doc = &.{.{ .key = "$sum", .value = .{ .string = "$n" } }} } },
+    } } }} };
+    const stages = [_]bson.Value{ by_k, count_groups };
+
+    var ctx = tdb.ctx(io);
+    var msg = try parse_fake_msg("aggregate", .{ .string = "gg" }, &.{
+        .{ .key = "pipeline", .value = .{ .array = &stages } },
+        .{ .key = "cursor", .value = .{ .doc = &.{} } },
+    });
+    defer msg.deinit();
+    var reply = wire.Reply.init(testing.allocator);
+    defer reply.deinit();
+    try dispatch(&ctx, &msg, &reply);
+
+    try testing.expectEqual(@as(f64, 1.0), bson.get_pair(reply.pairs.items, "ok").?.double);
+    const cur = bson.get_pair(reply.pairs.items, "cursor").?;
+    const batch = bson.get_pair(cur.doc, "firstBatch").?.array;
+    try testing.expectEqual(@as(usize, 1), batch.len);
+    try testing.expectEqual(@as(i32, 2), bson.get_pair(batch[0].doc, "groups").?.int32);
+    try testing.expectEqual(@as(i32, 3), bson.get_pair(batch[0].doc, "docs").?.int32);
+}
+
 test "$group refuses what it cannot compute instead of answering zero" {
     // The failure this closes was not a missing feature, it was a wrong number
     // reported as success. `{$avg: "$x"}` answered `0`, and so did `$max` and