diff --git a/nsparse/inverted_index.h b/nsparse/inverted_index.h
index b114292..10d08ad 100644
--- a/nsparse/inverted_index.h
+++ b/nsparse/inverted_index.h
@@ -34,6 +34,23 @@ class InvertedIndex : public MmapIndex, public IndexIO {
              const float* values) override;
     void build() override;
     size_t num_vectors() const override { return num_vectors_; }
+
+    // read_mcsr (the kMmap path) borrows vectors_ from the mapping without
+    // going through add(), which is otherwise the only thing besides build()
+    // that maintains num_vectors_. IDMapIndex::read_csr_and_ids cross-checks
+    // this count against the id file's as soon as the delegate's read_csr
+    // returns -- before build() runs -- so a mapped build reported 0 vectors
+    // and every id map was rejected. Sync it here, as DiskSeismicIndexBase
+    // does. (kInMemory routes through add(), which already set it, so
+    // re-reading get_vectors() is then a harmless no-op.)
+    void read_csr(const char* file_path,
+                  Residency residency = Residency::kInMemory) override {
+        MmapIndex::read_csr(file_path, residency);
+        const auto* vectors = get_vectors();
+        if (vectors != nullptr) {
+            num_vectors_ = vectors->num_vectors();
+        }
+    }
     std::array<char, 4> id() const override { return name; }
     static constexpr std::array<char, 4> name = {'I', 'N', 'V', 'T'};
     // Bump whenever write_index's payload layout changes.
diff --git a/tests/id_map_index_test.cpp b/tests/id_map_index_test.cpp
index 1014e56..2ab3930 100644
--- a/tests/id_map_index_test.cpp
+++ b/tests/id_map_index_test.cpp
@@ -565,6 +565,49 @@ TEST(IDMapReadCsrAndId, MatchesAddWithIdsBuildQuantized) {
     }
 }
 
+// read_csr_and_ids over an UNQUANTIZED, non-seismic delegate. InvertedIndex
+// overrides num_vectors() with a member that only add() and build() maintain,
+// so the mapped path -- which populates vectors_ without going through add() --
+// left it at 0 and read_csr_and_ids rejected every id map with a count
+// mismatch. The build is bit-exact to add_with_ids over the same corpus.
+TEST(IDMapReadCsrAndId, MatchesAddWithIdsBuildInverted) {
+    const Corpus corpus = make_corpus(300, kDim, /*seed=*/1);
+    const Corpus queries = make_corpus(20, kDim, /*seed=*/2);
+    const std::vector<idx_t> ids = make_external_ids(corpus.n);
+    constexpr int k = 10;
+
+    // Reference: add_with_ids() then build().
+    nsparse::IDMapIndex added(new nsparse::InvertedIndex(kDim));
+    added.add_with_ids(corpus.n, corpus.indptr.data(), corpus.indices.data(),
+                       corpus.values.data(), ids.data());
+    added.build();
+    const auto expected = search_corpus(added, queries, k);
+
+    // Under test: the same corpus borrowed from a mapped native CSR. Values
+    // stay float here, unlike the quantized case -- an inverted index reports
+    // the default 4-byte code_element_size and searches over floats.
+    nsparse::csr_test::TempCsrFiles csr("nsparse_idmap_inverted_src");
+    nsparse::csr_test::write_interchange_csr(csr.interchange(), corpus, kDim);
+    nsparse::csr_layout::convert(csr.interchange(), csr.native());
+    TempIdFile idfile("nsparse_idmap_inverted_src.ids");
+    nsparse::csr_test::write_id_map_file(idfile.path(), ids);
+
+    nsparse::IDMapIndex mapped(new nsparse::InvertedIndex(kDim));
+    mapped.read_csr_and_ids(csr.native().c_str(), idfile.path().c_str(),
+                            nsparse::Residency::kMmap);
+    // The count read_csr_and_ids checks the id file against, and what used to
+    // be 0 here.
+    ASSERT_EQ(mapped.num_vectors(), static_cast<size_t>(corpus.n));
+    mapped.build();
+    const auto got = search_corpus(mapped, queries, k);
+
+    EXPECT_EQ(got.first, expected.first) << "external-id labels differ";
+    ASSERT_EQ(got.second.size(), expected.second.size());
+    for (size_t i = 0; i < got.second.size(); ++i) {
+        EXPECT_FLOAT_EQ(got.second[i], expected.second[i]) << "score at " << i;
+    }
+}
+
 // The id map is row-aligned with the CSR, so a count that disagrees with the
 // delegate's vector count is rejected.
 TEST(IDMapReadCsrAndId, CountMismatchThrows) {