diff --git a/crates/polars-io/src/csv/read/options.rs b/crates/polars-io/src/csv/read/options.rs
index 3741bd6d9e47..2764d085d093 100644
--- a/crates/polars-io/src/csv/read/options.rs
+++ b/crates/polars-io/src/csv/read/options.rs
@@ -133,17 +133,6 @@ pub(super) enum NullValuesCompiled {
 }
 
 impl NullValuesCompiled {
-    pub(super) fn apply_projection(&mut self, projections: &[usize]) {
-        if let Self::Columns(nv) = self {
-            let nv = projections
-                .iter()
-                .map(|i| std::mem::take(&mut nv[*i]))
-                .collect::<Vec<_>>();
-
-            *self = NullValuesCompiled::Columns(nv);
-        }
-    }
-
     /// # Safety
     ///
     /// The caller must ensure that `index` is in bounds
diff --git a/crates/polars-io/src/csv/read/parser.rs b/crates/polars-io/src/csv/read/parser.rs
index 29845c990621..eff02dd4f19e 100644
--- a/crates/polars-io/src/csv/read/parser.rs
+++ b/crates/polars-io/src/csv/read/parser.rs
@@ -511,7 +511,7 @@ pub(super) fn parse_lines(
 
                             // SAFETY:
                             // process fields is in bounds
-                            add_null = unsafe { null_values.is_null(field, processed_fields) }
+                            add_null = unsafe { null_values.is_null(field, idx as usize) }
                         }
                         if add_null {
                             buf.add_null(!missing_is_null && field.is_empty())
diff --git a/crates/polars-io/src/csv/read/read_impl.rs b/crates/polars-io/src/csv/read/read_impl.rs
index a44c2ba8c90e..5805d1898fdc 100644
--- a/crates/polars-io/src/csv/read/read_impl.rs
+++ b/crates/polars-io/src/csv/read/read_impl.rs
@@ -224,8 +224,8 @@ impl<'a> CoreReader<'a> {
             }
         }
 
-        // create a null value for every column
-        let mut null_values = null_values.map(|nv| nv.compile(&schema)).transpose()?;
+        // Create a null value for every column
+        let null_values = null_values.map(|nv| nv.compile(&schema)).transpose()?;
 
         if let Some(cols) = columns {
             let mut prj = Vec::with_capacity(cols.len());
@@ -233,12 +233,6 @@ impl<'a> CoreReader<'a> {
                 let i = schema.try_index_of(&col)?;
                 prj.push(i);
             }
-
-            // update null values with projection
-            if let Some(nv) = null_values.as_mut() {
-                nv.apply_projection(&prj);
-            }
-
             projection = Some(prj);
         }
 
diff --git a/py-polars/tests/unit/io/test_lazy_csv.py b/py-polars/tests/unit/io/test_lazy_csv.py
index 59bb84d72658..59e7291ea522 100644
--- a/py-polars/tests/unit/io/test_lazy_csv.py
+++ b/py-polars/tests/unit/io/test_lazy_csv.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import tempfile
 from collections import OrderedDict
 from typing import TYPE_CHECKING
 
@@ -285,3 +286,25 @@ def test_scan_empty_csv_with_row_index(tmp_path: Path) -> None:
 
     read = pl.scan_csv(file_path).with_row_index("idx")
     assert read.collect().schema == OrderedDict([("idx", pl.UInt32), ("a", pl.String)])
+
+
+@pytest.mark.write_disk()
+def test_csv_null_values_with_projection_15515() -> None:
+    data = """IndCode,SireCode,BirthDate,Flag
+ID00316,.,19940315,
+"""
+
+    with tempfile.NamedTemporaryFile() as f:
+        f.write(data.encode())
+        f.seek(0)
+
+        q = (
+            pl.scan_csv(f.name, null_values={"SireCode": "."})
+            .with_columns(pl.col("SireCode").alias("SireKey"))
+            .select("SireKey", "BirthDate")
+        )
+
+        assert q.collect().to_dict(as_series=False) == {
+            "SireKey": [None],
+            "BirthDate": [19940315],
+        }