diff --git a/dbms/src/Storages/AlterCommands.cpp b/dbms/src/Storages/AlterCommands.cpp
index 2d75d7ab840..8c2244b9aa1 100644
--- a/dbms/src/Storages/AlterCommands.cpp
+++ b/dbms/src/Storages/AlterCommands.cpp
@@ -765,8 +765,7 @@ void AlterCommands::validateDefaultExpressionForColumn(
         auto default_with_cast = makeASTFunction("CAST", copy_expression, std::make_shared<ASTLiteral>(column_type->getName()));
         auto query_with_alias = setAlias(default_with_cast, tmp_column_name);
         auto syntax_result = SyntaxAnalyzer(context).analyze(query_with_alias, all_columns.getAll());
-        const auto actions = ExpressionAnalyzer(query_with_alias, syntax_result, context).getActions(true);
-        auto sample_block = actions->getSampleBlock();
+        ExpressionAnalyzer(query_with_alias, syntax_result, context).getActions(true);
     }
     catch (Exception & ex)
     {
diff --git a/dbms/src/Storages/AlterCommands.h b/dbms/src/Storages/AlterCommands.h
index 2bae2d3465d..06c6ff6e23f 100644
--- a/dbms/src/Storages/AlterCommands.h
+++ b/dbms/src/Storages/AlterCommands.h
@@ -131,6 +131,8 @@ private:
     bool prepared = false;
 private:
 
+    /// Validate that default expression and type are compatible, i.e. default
+    /// expression result can be casted to column_type
     void validateDefaultExpressionForColumn(
         const ASTPtr default_expression,
         const String & column_name,
@@ -145,8 +147,8 @@ public:
     /// More accurate check have to be performed with storage->checkAlterIsPossible.
     void validate(const StorageInMemoryMetadata & metadata, const Context & context) const;
 
-    /// Prepare alter commands. Set ignore flag to some of them
-    /// and additional commands for dependent columns.
+    /// Prepare alter commands. Set ignore flag to some of them and set some
+    /// parts to commands from storage's metadata (for example, absent default)
     void prepare(const StorageInMemoryMetadata & metadata, const Context & context);
 
     /// Apply all alter command in sequential order to storage metadata.