diff --git a/src/function/cast/vector_cast_helpers.cpp b/src/function/cast/vector_cast_helpers.cpp
index a064b4aca3e..c7aa523eaa0 100644
--- a/src/function/cast/vector_cast_helpers.cpp
+++ b/src/function/cast/vector_cast_helpers.cpp
@@ -4,7 +4,9 @@ namespace duckdb {
 
 // ------- Helper functions for splitting string nested types  -------
 static bool IsNull(const char *buf, idx_t start_pos, Vector &child, idx_t row_idx) {
-	if (buf[start_pos] == 'N' && buf[start_pos + 1] == 'U' && buf[start_pos + 2] == 'L' && buf[start_pos + 3] == 'L') {
+	if ((buf[start_pos] == 'N' || buf[start_pos] == 'n') && (buf[start_pos + 1] == 'U' || buf[start_pos + 1] == 'u') &&
+	    (buf[start_pos + 2] == 'L' || buf[start_pos + 2] == 'l') &&
+	    (buf[start_pos + 3] == 'L' || buf[start_pos + 3] == 'l')) {
 		FlatVector::SetNull(child, row_idx, true);
 		return true;
 	}
diff --git a/test/sql/types/struct/struct_cast.test b/test/sql/types/struct/struct_cast.test
index 91303e432a0..2046e00a392 100644
--- a/test/sql/types/struct/struct_cast.test
+++ b/test/sql/types/struct/struct_cast.test
@@ -112,3 +112,43 @@ SELECT s::ROW(i BIGINT, j ROW(a BIGINT, b VARCHAR)) FROM nested_structs
 {'i': 1, 'j': {'a': 2, 'b': NULL}}
 {'i': 1, 'j': NULL}
 NULL
+
+# Issue #12939, NULL used to be case-sensitive
+# Testing all permutations for fun
+query I
+SELECT col::STRUCT(duck INT)
+FROM VALUES
+    ('{"duck": null}'),
+    ('{"duck": nulL}'),
+    ('{"duck": nuLl}'),
+    ('{"duck": nuLL}'),
+    ('{"duck": nUll}'),
+    ('{"duck": nUlL}'),
+    ('{"duck": nULl}'),
+    ('{"duck": nULL}'),
+    ('{"duck": Null}'),
+    ('{"duck": NulL}'),
+    ('{"duck": NuLl}'),
+    ('{"duck": NuLL}'),
+    ('{"duck": NUll}'),
+    ('{"duck": NUlL}'),
+    ('{"duck": NULl}'),
+    ('{"duck": NULL}'),
+AS tab(col)
+----
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}
+{'duck': NULL}