diff --git a/src/common/base/Base.h b/src/common/base/Base.h
index 0f85a45702b..501cfdc19f5 100644
--- a/src/common/base/Base.h
+++ b/src/common/base/Base.h
@@ -156,11 +156,11 @@ using VariantType = boost::variant<int64_t, double, bool, std::string>;
 #endif
 
 // reserved property names
-constexpr char _ID[] = "_id";
-constexpr char _SRC[] = "_src";
-constexpr char _TYPE[] = "_type";
-constexpr char _RANK[] = "_rank";
-constexpr char _DST[] = "_dst";
+constexpr char _ID[]    = "_id";
+constexpr char _SRC[]   = "_src";
+constexpr char _TYPE[]  = "_type";
+constexpr char _RANK[]  = "_rank";
+constexpr char _DST[]   = "_dst";
 // Useful type traits
 
 // Tell if `T' is copy-constructible
diff --git a/src/common/filter/Expressions.cpp b/src/common/filter/Expressions.cpp
index da6b6257d54..4eaa4dbf1e7 100644
--- a/src/common/filter/Expressions.cpp
+++ b/src/common/filter/Expressions.cpp
@@ -110,7 +110,7 @@ std::string AliasPropertyExpression::toString() const {
     std::string buf;
     buf.reserve(64);
     buf += *ref_;
-    if (*ref_ != "" && *ref_ != "$") {
+    if (*ref_ != "" && *ref_ != VAR_REF) {
         buf += ".";
     }
     buf += *alias_;
diff --git a/src/graph/test/YieldTest.cpp b/src/graph/test/YieldTest.cpp
index ba72403c644..850dcbd5849 100644
--- a/src/graph/test/YieldTest.cpp
+++ b/src/graph/test/YieldTest.cpp
@@ -412,6 +412,7 @@ TEST_F(YieldTest, yieldVar) {
 TEST_F(YieldTest, error) {
     {
         cpp2::ExecutionResponse resp;
+        // Reference input in a single yield sentence is meaningless.
         auto query = "yield $-";
         auto code = client_->execute(query, resp);
         UNUSED(code);
@@ -421,6 +422,7 @@ TEST_F(YieldTest, error) {
     {
         cpp2::ExecutionResponse resp;
         auto &player = players_["Boris Diaw"];
+        // Not support reference input and variable
         auto fmt = var + "YIELD $var.team WHERE $-.start > 2005";
         auto query = folly::stringPrintf(fmt.c_str(), player.vid());
         auto code = client_->execute(query, resp);
@@ -429,6 +431,7 @@ TEST_F(YieldTest, error) {
     {
         cpp2::ExecutionResponse resp;
         auto &player = players_["Boris Diaw"];
+        // Not support reference two diffrent variable
         auto fmt = var + "YIELD $var.team WHERE $var1.start > 2005";
         auto query = folly::stringPrintf(fmt.c_str(), player.vid());
         auto code = client_->execute(query, resp);
@@ -437,6 +440,7 @@ TEST_F(YieldTest, error) {
     {
         cpp2::ExecutionResponse resp;
         auto &player = players_["Boris Diaw"];
+        // Reference properties in single yield sentence is meaningless.
         auto fmt = var + "YIELD $$.a.team";
         auto query = folly::stringPrintf(fmt.c_str(), player.vid());
         auto code = client_->execute(query, resp);