diff --git a/src/query.zig b/src/query.zig
index 18f799b..c91e11b 100644
--- a/src/query.zig
+++ b/src/query.zig
@@ -182,12 +182,42 @@ fn apply_expected(
         return false;
     }
     // Bare equality — matches if any candidate equals the expected value.
-    for (candidates) |actual| {
+    for (equality_candidates(candidates)) |actual| {
         if (bson.compare(actual, expected) == .eq) return true;
     }
     return false;
 }
 
+/// A path that yields nothing is null to the equality family, and to it
+/// alone.
+///
+/// `{a: null}` matches a document with no `a` at all, as well as one holding
+/// an explicit null. `{a: {$lt: 5}}` does not, even though null sorts below
+/// 5; `{a: {$exists: false}}` has to keep seeing that there is nothing there;
+/// `{a: {$type: "null"}}` stays false. So this substitutes a single null
+/// rather than collecting one, and only where equality can see it.
+///
+/// The other two layers have always agreed with this and only the matcher
+/// did not: `index.build_entries` stores a missing field as null under a
+/// non-sparse index, and the planner's sparse guard exists to stop exactly
+/// this query reading an index that skipped those documents. Without this the
+/// guard was defending a behaviour that did not exist.
+const missing_as_null = [_]bson.Value{.null};
+
+fn equality_candidates(candidates: []const bson.Value) []const bson.Value {
+    return if (candidates.len == 0) &missing_as_null else candidates;
+}
+
+/// Whether `op` compares for equality, which is what decides that a missing
+/// field is null. `$ne` and `$nin` are in it because they are the negations:
+/// `{a: {$ne: null}}` has to *exclude* a document with no `a`.
+fn equality_family(op: Op) bool {
+    return switch (op) {
+        .eq, .ne, .in, .nin => true,
+        else => false,
+    };
+}
+
 /// Whether a stored document (canonical BSON bytes) matches `filter` — the
 /// byte-matcher counterpart of `matches`, used by scans. Same semantics,
 /// different collection: fields the filter does not name are skipped by
@@ -398,9 +428,10 @@ fn match_operator(
     gpa: std.mem.Allocator,
     op: Op,
     value: bson.Value,
-    actuals: []const bson.Value,
+    actuals_at_path: []const bson.Value,
     regex_options: []const u8,
 ) QueryError!bool {
+    const actuals = if (equality_family(op)) equality_candidates(actuals_at_path) else actuals_at_path;
     if (op == .eq) {
         for (actuals) |a| if (bson.compare(a, value) == .eq) return true;
         return false;
@@ -1348,6 +1379,95 @@ test "array index dot path and bare regex value" {
     try testing.expect(!try matches(testing.allocator, &doc_of(&.{.{ .key = "name", .value = .{ .regex = .{ .pattern = "^z", .options = "" } } }}), &d));
 }
 
+test "a missing field is null to equality and to nothing else" {
+    const gpa = testing.allocator;
+    // No `a` at all. The index layer has always stored this document under
+    // null in a non-sparse index; the matcher used to disagree, so
+    // `find({a: null})` answered nothing here and two documents on mongod.
+    const d = doc_of(&.{ .{ .key = "_id", .value = .{ .int32 = 1 } }, .{ .key = "z", .value = .{ .int32 = 1 } } });
+
+    // A table of static filters: `&.{...}` inside a `const` initializer is a
+    // comptime constant, where the same thing returned from a helper would be
+    // a pointer into that helper's dead frame.
+    const Case = struct { what: []const u8, filter: []const bson.Pair, want: bool };
+    const cases = [_]Case{
+        // The equality family sees a null...
+        .{ .what = "{a: null}", .want = true, .filter = &.{
+            .{ .key = "a", .value = .null },
+        } },
+        .{ .what = "$eq null", .want = true, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$eq", .value = .null }} } },
+        } },
+        .{ .what = "$in [null, 1]", .want = true, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$in", .value = .{ .array = &.{
+                .null,
+                .{ .int32 = 1 },
+            } } }} } },
+        } },
+        // ...including through a dotted path that stops short...
+        .{ .what = "{a.b: null}", .want = true, .filter = &.{
+            .{ .key = "a.b", .value = .null },
+        } },
+        // ...and the negations exclude it, which is the half that a bare
+        // "true when there are no candidates" would get backwards.
+        .{ .what = "$ne null", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$ne", .value = .null }} } },
+        } },
+        .{ .what = "$nin [null]", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$nin", .value = .{ .array = &.{.null} } }} } },
+        } },
+        // A non-null equality still finds nothing.
+        .{ .what = "{a: 1}", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .int32 = 1 } },
+        } },
+        .{ .what = "$ne 1", .want = true, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$ne", .value = .{ .int32 = 1 } }} } },
+        } },
+        // Nothing outside the family sees it. Mutation check: widen
+        // `equality_family` to every operator, or collect the null in
+        // `field_matches` instead of substituting it here, and these flip.
+        .{ .what = "$lt 5", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$lt", .value = .{ .int32 = 5 } }} } },
+        } },
+        .{ .what = "$gte null", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$gte", .value = .null }} } },
+        } },
+        .{ .what = "$type null", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$type", .value = .{ .string = "null" } }} } },
+        } },
+        .{ .what = "$size 0", .want = false, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$size", .value = .{ .int32 = 0 } }} } },
+        } },
+        .{ .what = "$exists false", .want = true, .filter = &.{
+            .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$exists", .value = .{ .bool = false } }} } },
+        } },
+    };
+
+    // The byte matcher answers scans and the tree matcher answers everything
+    // else, so both are run over every case: a rule in one and not the other
+    // would show up as a filter that changes its mind after a checkpoint.
+    var bytes: std.ArrayListUnmanaged(u8) = .empty;
+    defer bytes.deinit(gpa);
+    try bson.write_doc(d.pairs, gpa, &bytes);
+    for (cases) |c| {
+        errdefer std.debug.print("case: {s}\n", .{c.what});
+        try testing.expectEqual(c.want, try matches(gpa, &doc_of(c.filter), &d));
+        try testing.expectEqual(c.want, try matches_bytes(gpa, c.filter, bytes.items));
+    }
+
+    // An explicit null is unaffected, and so is a value.
+    const eq_null: []const bson.Pair = &.{.{ .key = "a", .value = .null }};
+    const ne_null: []const bson.Pair = &.{
+        .{ .key = "a", .value = .{ .doc = &.{.{ .key = "$ne", .value = .null }} } },
+    };
+    const explicit = doc_of(&.{.{ .key = "a", .value = .null }});
+    try testing.expect(try matches(gpa, &doc_of(eq_null), &explicit));
+    try testing.expect(!try matches(gpa, &doc_of(ne_null), &explicit));
+    const valued = doc_of(&.{.{ .key = "a", .value = .{ .int32 = 1 } }});
+    try testing.expect(!try matches(gpa, &doc_of(eq_null), &valued));
+    try testing.expect(try matches(gpa, &doc_of(ne_null), &valued));
+}
+
 test "sort compares by BSON order" {
     const a = doc_of(&.{ .{ .key = "n", .value = .{ .int32 = 2 } }, .{ .key = "x", .value = .{ .string = "a" } } });
     const b = doc_of(&.{ .{ .key = "n", .value = .{ .int32 = 10 } }, .{ .key = "x", .value = .{ .string = "b" } } });