diff --git a/CHANGELOG.md b/CHANGELOG.md
index 98841a11..f0ada28d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ This changelog documents the changes between release versions.
 - Implement column-to-column comparisons within the same collection ([#74](https://github.com/hasura/ndc-mongodb/pull/74))
 - Fix error tracking collection with no documents by skipping such collections during CLI introspection ([#76](https://github.com/hasura/ndc-mongodb/pull/76))
 - If a field contains both `int` and `double` values then the field type is inferred as `double` instead of `ExtendedJSON` ([#77](https://github.com/hasura/ndc-mongodb/pull/77))
+- Fix: schema generated with `_id` column nullable when introspecting schema via sampling ([#78](https://github.com/hasura/ndc-mongodb/pull/78))
 
 ## [0.0.6] - 2024-05-01
 - Enables logging events from the MongoDB driver by setting the `RUST_LOG` variable ([#67](https://github.com/hasura/ndc-mongodb/pull/67))
diff --git a/crates/cli/src/introspection/sampling.rs b/crates/cli/src/introspection/sampling.rs
index 47e3dec6..51a5f720 100644
--- a/crates/cli/src/introspection/sampling.rs
+++ b/crates/cli/src/introspection/sampling.rs
@@ -21,7 +21,7 @@ type ObjectType = WithName<schema::ObjectType>;
 /// are not unifiable.
 pub async fn sample_schema_from_db(
     sample_size: u32,
-    all_schema_nullalble: bool,
+    all_schema_nullable: bool,
     config_file_changed: bool,
     state: &ConnectorState,
     existing_schemas: &HashSet<std::string::String>,
@@ -36,7 +36,7 @@ pub async fn sample_schema_from_db(
             let collection_schema = sample_schema_from_collection(
                 &collection_name,
                 sample_size,
-                all_schema_nullalble,
+                all_schema_nullable,
                 state,
             )
             .await?;
@@ -53,7 +53,7 @@ pub async fn sample_schema_from_db(
 async fn sample_schema_from_collection(
     collection_name: &str,
     sample_size: u32,
-    all_schema_nullalble: bool,
+    all_schema_nullable: bool,
     state: &ConnectorState,
 ) -> anyhow::Result<Option<Schema>> {
     let db = state.database();
@@ -63,8 +63,14 @@ async fn sample_schema_from_collection(
         .aggregate(vec![doc! {"$sample": { "size": sample_size }}], options)
         .await?;
     let mut collected_object_types = vec![];
+    let is_collection_type = true;
     while let Some(document) = cursor.try_next().await? {
-        let object_types = make_object_type(collection_name, &document, all_schema_nullalble);
+        let object_types = make_object_type(
+            collection_name,
+            &document,
+            is_collection_type,
+            all_schema_nullable,
+        );
         collected_object_types = if collected_object_types.is_empty() {
             object_types
         } else {
@@ -91,14 +97,21 @@ async fn sample_schema_from_collection(
 fn make_object_type(
     object_type_name: &str,
     document: &Document,
-    all_schema_nullalble: bool,
+    is_collection_type: bool,
+    all_schema_nullable: bool,
 ) -> Vec<ObjectType> {
     let (mut object_type_defs, object_fields) = {
         let type_prefix = format!("{object_type_name}_");
         let (object_type_defs, object_fields): (Vec<Vec<ObjectType>>, Vec<ObjectField>) = document
             .iter()
             .map(|(field_name, field_value)| {
-                make_object_field(&type_prefix, field_name, field_value, all_schema_nullalble)
+                make_object_field(
+                    &type_prefix,
+                    field_name,
+                    field_value,
+                    is_collection_type,
+                    all_schema_nullable,
+                )
             })
             .unzip();
         (object_type_defs.concat(), object_fields)
@@ -120,11 +133,12 @@ fn make_object_field(
     type_prefix: &str,
     field_name: &str,
     field_value: &Bson,
-    all_schema_nullalble: bool,
+    is_collection_type: bool,
+    all_schema_nullable: bool,
 ) -> (Vec<ObjectType>, ObjectField) {
     let object_type_name = format!("{type_prefix}{field_name}");
     let (collected_otds, field_type) =
-        make_field_type(&object_type_name, field_value, all_schema_nullalble);
+        make_field_type(&object_type_name, field_value, all_schema_nullable);
     let object_field_value = WithName::named(
         field_name.to_owned(),
         schema::ObjectField {
@@ -132,7 +146,8 @@ fn make_object_field(
             r#type: field_type,
         },
     );
-    let object_field = if all_schema_nullalble {
+    let object_field = if all_schema_nullable && !(is_collection_type && field_name == "_id") {
+        // The _id field on a collection type should never be nullable.
         make_nullable_field(object_field_value)
     } else {
         object_field_value
@@ -145,16 +160,16 @@ fn make_object_field(
 pub fn type_from_bson(
     object_type_name: &str,
     value: &Bson,
-    all_schema_nullalble: bool,
+    all_schema_nullable: bool,
 ) -> (BTreeMap<std::string::String, schema::ObjectType>, Type) {
-    let (object_types, t) = make_field_type(object_type_name, value, all_schema_nullalble);
+    let (object_types, t) = make_field_type(object_type_name, value, all_schema_nullable);
     (WithName::into_map(object_types), t)
 }
 
 fn make_field_type(
     object_type_name: &str,
     field_value: &Bson,
-    all_schema_nullalble: bool,
+    all_schema_nullable: bool,
 ) -> (Vec<ObjectType>, Type) {
     fn scalar(t: BsonScalarType) -> (Vec<ObjectType>, Type) {
         (vec![], Type::Scalar(t))
@@ -168,7 +183,7 @@ fn make_field_type(
             let mut result_type = Type::Scalar(Undefined);
             for elem in arr {
                 let (elem_collected_otds, elem_type) =
-                    make_field_type(object_type_name, elem, all_schema_nullalble);
+                    make_field_type(object_type_name, elem, all_schema_nullable);
                 collected_otds = if collected_otds.is_empty() {
                     elem_collected_otds
                 } else {
@@ -179,7 +194,13 @@ fn make_field_type(
             (collected_otds, Type::ArrayOf(Box::new(result_type)))
         }
         Bson::Document(document) => {
-            let collected_otds = make_object_type(object_type_name, document, all_schema_nullalble);
+            let is_collection_type = false;
+            let collected_otds = make_object_type(
+                object_type_name,
+                document,
+                is_collection_type,
+                all_schema_nullable,
+            );
             (collected_otds, Type::Object(object_type_name.to_owned()))
         }
         Bson::Boolean(_) => scalar(Bool),
@@ -220,7 +241,7 @@ mod tests {
         let object_name = "foo";
         let doc = doc! {"my_int": 1, "my_string": "two"};
         let result =
-            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false));
+            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false, false));
 
         let expected = BTreeMap::from([(
             object_name.to_owned(),
@@ -250,12 +271,54 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn simple_doc_nullable_fields() -> Result<(), anyhow::Error> {
+        let object_name = "foo";
+        let doc = doc! {"my_int": 1, "my_string": "two", "_id": 0};
+        let result =
+            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, true, true));
+
+        let expected = BTreeMap::from([(
+            object_name.to_owned(),
+            ObjectType {
+                fields: BTreeMap::from([
+                    (
+                        "_id".to_owned(),
+                        ObjectField {
+                            r#type: Type::Scalar(BsonScalarType::Int),
+                            description: None,
+                        },
+                    ),
+                    (
+                        "my_int".to_owned(),
+                        ObjectField {
+                            r#type: Type::Nullable(Box::new(Type::Scalar(BsonScalarType::Int))),
+                            description: None,
+                        },
+                    ),
+                    (
+                        "my_string".to_owned(),
+                        ObjectField {
+                            r#type: Type::Nullable(Box::new(Type::Scalar(BsonScalarType::String))),
+                            description: None,
+                        },
+                    ),
+                ]),
+                description: None,
+            },
+        )]);
+
+        assert_eq!(expected, result);
+
+        Ok(())
+    }
+
     #[test]
     fn array_of_objects() -> Result<(), anyhow::Error> {
         let object_name = "foo";
         let doc = doc! {"my_array": [{"foo": 42, "bar": ""}, {"bar": "wut", "baz": 3.77}]};
         let result =
-            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false));
+            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false, false));
 
         let expected = BTreeMap::from([
             (
@@ -316,7 +379,7 @@ mod tests {
         let object_name = "foo";
         let doc = doc! {"my_array": [{"foo": 42, "bar": ""}, {"bar": 17, "baz": 3.77}]};
         let result =
-            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false));
+            WithName::into_map::<BTreeMap<_, _>>(make_object_type(object_name, &doc, false, false));
 
         let expected = BTreeMap::from([
             (
diff --git a/crates/cli/src/lib.rs b/crates/cli/src/lib.rs
index 34622108..46b510a5 100644
--- a/crates/cli/src/lib.rs
+++ b/crates/cli/src/lib.rs
@@ -56,7 +56,7 @@ async fn update(context: &Context, args: &UpdateArgs) -> anyhow::Result<()> {
         None => configuration_options.introspection_options.no_validator_schema
     };
     let all_schema_nullable = match args.all_schema_nullable {
-        Some(validator) => validator,
+        Some(b) => b,
         None => configuration_options.introspection_options.all_schema_nullable
     };
     let config_file_changed = configuration::get_config_file_changed(&context.path).await?;
diff --git a/crates/configuration/src/configuration.rs b/crates/configuration/src/configuration.rs
index 8c645515..e7719ec7 100644
--- a/crates/configuration/src/configuration.rs
+++ b/crates/configuration/src/configuration.rs
@@ -206,7 +206,7 @@ pub struct ConfigurationIntrospectionOptions {
     // Whether to try validator schema first if one exists.
     pub no_validator_schema: bool,
 
-    // Default to setting all schema fields as nullable.
+    // Default to setting all schema fields, except the _id field on collection types, as nullable.
     pub all_schema_nullable: bool,
 }