diff --git a/internal/core/src/mmap/ChunkedColumn.h b/internal/core/src/mmap/ChunkedColumn.h
index d4e38d2f90..8368877217 100644
--- a/internal/core/src/mmap/ChunkedColumn.h
+++ b/internal/core/src/mmap/ChunkedColumn.h
@@ -175,6 +175,10 @@ class ChunkedColumnBase : public ChunkedColumnInterface {
                     fn(true, i);
                 }
             }
+            // Non-nullable is fully handled above; without this return the
+            // control falls through into the nullable block and invokes fn a
+            // second time per row.
+            return;
         }
         // nullable:
         if (offsets == nullptr) {
diff --git a/internal/core/src/mmap/ChunkedColumnGroup.h b/internal/core/src/mmap/ChunkedColumnGroup.h
index a5b3d1b2bf..f235c7f7e5 100644
--- a/internal/core/src/mmap/ChunkedColumnGroup.h
+++ b/internal/core/src/mmap/ChunkedColumnGroup.h
@@ -269,6 +269,10 @@ class ProxyChunkColumn : public ChunkedColumnInterface {
                     fn(true, i);
                 }
             }
+            // Non-nullable is fully handled above; without this return the
+            // control falls through into the nullable block and invokes fn a
+            // second time per row.
+            return;
         }
         // nullable:
         if (count == 0) {
diff --git a/internal/core/src/mmap/ChunkedColumnGroupTest.cpp b/internal/core/src/mmap/ChunkedColumnGroupTest.cpp
index d653eb9ef3..4c0143c38b 100644
--- a/internal/core/src/mmap/ChunkedColumnGroupTest.cpp
+++ b/internal/core/src/mmap/ChunkedColumnGroupTest.cpp
@@ -274,6 +274,29 @@ TEST_F(ChunkedColumnGroupTest, ProxyChunkColumn) {
         &offset,
         1);
 
+    // Regression: a non-nullable column must invoke the callback EXACTLY once
+    // per row. Before the missing-return fix, the !nullable_ branch fell
+    // through into the nullable branch and invoked the callback a second time
+    // per row.
+    {
+        std::vector<int64_t> all_offsets = {0, 1, 2, 3, 4};
+        std::unordered_map<size_t, int> call_count;
+        proxy_int64->BulkIsValid(
+            nullptr,
+            [&](bool is_valid, size_t i) {
+                EXPECT_TRUE(is_valid);
+                call_count[i]++;
+            },
+            all_offsets.data(),
+            static_cast<int64_t>(all_offsets.size()));
+        ASSERT_EQ(call_count.size(), all_offsets.size());
+        for (const auto& kv : call_count) {
+            EXPECT_EQ(kv.second, 1)
+                << "row " << kv.first << " callback invoked " << kv.second
+                << " times";
+        }
+    }
+
     // Test string proxy
     auto proxy_string = std::make_shared<ProxyChunkColumn>(
         column_group, FieldId(2), string_field_meta);