diff --git a/src/Functions/padString.cpp b/src/Functions/padString.cpp
index 7a424bb1198..119baaec723 100644
--- a/src/Functions/padString.cpp
+++ b/src/Functions/padString.cpp
@@ -241,7 +241,7 @@ namespace
             bool all_ascii = UTF8::isAllASCII(reinterpret_cast<const UInt8 *>(pad_string.data()), pad_string.size())
                 && UTF8::isAllASCII(chars.data(), chars.size());
 
-            if (all_ascii)
+            if (!is_utf8 || all_ascii)
             {
                 PaddingChars<false> padding_chars{pad_string};
                 if (const auto * col_const = checkAndGetColumn<ColumnConst>(column_length.get()))
diff --git a/src/Functions/substring.cpp b/src/Functions/substring.cpp
index 759d41e2ab8..122f83d758b 100644
--- a/src/Functions/substring.cpp
+++ b/src/Functions/substring.cpp
@@ -161,9 +161,9 @@ public:
                 StringRef str_ref = col_const->getDataAt(0);
                 bool all_ascii = UTF8::isAllASCII(reinterpret_cast<const UInt8 *>(str_ref.data), str_ref.size);
                 if (all_ascii)
-                    return executeForSource(column_offset, column_length, column_offset_const, column_length_const, offset, length, ConstSource<UTF8StringSource>(*col_const), input_rows_count);
-                else
                     return executeForSource(column_offset, column_length, column_offset_const, column_length_const, offset, length, ConstSource<StringSource>(*col_const), input_rows_count);
+                else
+                    return executeForSource(column_offset, column_length, column_offset_const, column_length_const, offset, length, ConstSource<UTF8StringSource>(*col_const), input_rows_count);
             }
             throw Exception(ErrorCodes::ILLEGAL_COLUMN, "Illegal column {} of first argument of function {}", arguments[0].column->getName(), getName());
         }