diff --git a/FreeSql.Tests/FreeSql.Tests/UnitTest4.cs b/FreeSql.Tests/FreeSql.Tests/UnitTest4.cs
index d369cc98..4538707a 100644
--- a/FreeSql.Tests/FreeSql.Tests/UnitTest4.cs
+++ b/FreeSql.Tests/FreeSql.Tests/UnitTest4.cs
@@ -10,6 +10,41 @@ namespace FreeSql.Tests
 {
     public class UnitTest4
     {
+        [Fact]
+        public void LeftJoinNull01()
+        {
+            var fsql = g.sqlite;
+            fsql.Delete<leftjoin_null01>().Where("1=1").ExecuteAffrows();
+            fsql.Delete<leftjoin_null02>().Where("1=1").ExecuteAffrows();
+
+            var item = new leftjoin_null01 { name = "xx01" };
+            fsql.Insert(item).ExecuteAffrows();
+
+            var sel1 = fsql.Select<leftjoin_null01, leftjoin_null02>()
+                .LeftJoin((a, b) => a.id == b.null01_id)
+                .First((a, b) => new
+                {
+                    a.id,
+                    a.name,
+                    id2 = (Guid?)b.id,
+                    time2 = (DateTime?)b.time
+                });
+            Assert.Null(sel1.id2);
+            Assert.Null(sel1.time2);
+        }
+
+        class leftjoin_null01
+        {
+            public Guid id { get; set; }
+            public string name { get; set; }
+        }
+        class leftjoin_null02
+        {
+            public Guid id { get; set; }
+            public Guid null01_id { get; set; }
+            public DateTime time { get; set; }
+        }
+
 
         [Fact]
         public void TestHzyTuple()
diff --git a/FreeSql/Internal/CommonExpression.cs b/FreeSql/Internal/CommonExpression.cs
index 07739905..0952cf1f 100644
--- a/FreeSql/Internal/CommonExpression.cs
+++ b/FreeSql/Internal/CommonExpression.cs
@@ -178,14 +178,14 @@ namespace FreeSql.Internal
                             parent.Property = parentProp; //若不加此行，会引用 GroupBy(..).ToList(a => new Dto { key = a.Key }) null 错误，CopyTo 之后 Property 变为 null
                             return false;
                         }
-                        parent.CsType = exp.Type;
+                        if (parent.CsType == null) parent.CsType = exp.Type;
                         parent.DbField = ExpressionLambdaToSql(exp, getTSC());
                         field.Append(", ").Append(parent.DbField);
                         if (index >= 0) field.Append(_common.FieldAsAlias($"as{++index}"));
                         else if (index == ReadAnonymousFieldAsCsName && string.IsNullOrEmpty(parent.CsName) == false && 
                             parent.DbField.EndsWith(_common.QuoteSqlName(parent.CsName), StringComparison.CurrentCultureIgnoreCase) == false //DbField 和 CsName 相同的时候，不处理
                             ) field.Append(_common.FieldAsAlias(parent.CsName));
-                        parent.MapType = SearchColumnByField(_tables, null, parent.DbField)?.Attribute.MapType ?? exp.Type;
+                        if (parent.MapType == null) parent.MapType = SearchColumnByField(_tables, null, parent.DbField)?.Attribute.MapType ?? exp.Type;
                         return false;
                     }
                     return false;