diff --git a/dask_expr/_merge.py b/dask_expr/_merge.py
index cacc25c8..20e3fc81 100644
--- a/dask_expr/_merge.py
+++ b/dask_expr/_merge.py
@@ -9,6 +9,7 @@
 from dask_expr._expr import Blockwise, Expr, Index, PartitionsFiltered, Projection
 from dask_expr._repartition import Repartition
 from dask_expr._shuffle import AssignPartitioningIndex, Shuffle, _contains_index_name
+from dask_expr._util import _convert_to_list
 
 _HASH_COLUMN_NAME = "__hash_partition"
 
@@ -203,13 +204,20 @@ def _simplify_up(self, parent):
                     projection = [projection]
 
             left, right = self.left, self.right
-            left_on, right_on = self.left_on, self.right_on
+            left_on = _convert_to_list(self.left_on)
+            if left_on is None:
+                left_on = []
+
+            right_on = _convert_to_list(self.right_on)
+            if right_on is None:
+                right_on = []
+
             left_suffix, right_suffix = self.suffixes[0], self.suffixes[1]
             project_left, project_right = [], []
 
             # Find columns to project on the left
             for col in left.columns:
-                if left_on is not None and col in left_on or col in projection:
+                if col in left_on or col in projection:
                     project_left.append(col)
                 elif f"{col}{left_suffix}" in projection:
                     project_left.append(col)
@@ -220,7 +228,7 @@ def _simplify_up(self, parent):
 
             # Find columns to project on the right
             for col in right.columns:
-                if right_on is not None and col in right_on or col in projection:
+                if col in right_on or col in projection:
                     project_right.append(col)
                 elif f"{col}{right_suffix}" in projection:
                     project_right.append(col)
diff --git a/dask_expr/tests/test_merge.py b/dask_expr/tests/test_merge.py
index 96365473..dacb92b3 100644
--- a/dask_expr/tests/test_merge.py
+++ b/dask_expr/tests/test_merge.py
@@ -164,3 +164,15 @@ def test_merge_len():
     query = df.merge(df2).index.optimize(fuse=False)
     expected = df[["x"]].merge(df2[["x"]]).index.optimize(fuse=False)
     assert query._name == expected._name
+
+
+def test_merge_optimize_subset_strings():
+    pdf = lib.DataFrame({"a": [1, 2], "aaa": 1})
+    pdf2 = lib.DataFrame({"b": [1, 2], "aaa": 1})
+    df = from_pandas(pdf)
+    df2 = from_pandas(pdf2)
+
+    query = df.merge(df2, on="aaa")[["aaa"]].optimize(fuse=False)
+    exp = df[["aaa"]].merge(df2[["aaa"]], on="aaa").optimize(fuse=False)
+    assert query._name == exp._name
+    assert_eq(query, pdf.merge(pdf2, on="aaa")[["aaa"]])