diff --git a/src/Storages/AlterCommands.cpp b/src/Storages/AlterCommands.cpp
index d68252679a7..dcd7abae68a 100644
--- a/src/Storages/AlterCommands.cpp
+++ b/src/Storages/AlterCommands.cpp
@@ -755,9 +755,9 @@ bool isMetadataOnlyConversion(const IDataType * from, const IDataType * to)
 
         const auto * nullable_from = typeid_cast<const DataTypeNullable *>(from);
         const auto * nullable_to = typeid_cast<const DataTypeNullable *>(to);
-        if (nullable_to)
+        if (nullable_from && nullable_to)
         {
-            from = nullable_from ? nullable_from->getNestedType().get() : from;
+            from = nullable_from->getNestedType().get();
             to = nullable_to->getNestedType().get();
             continue;
         }