diff --git a/src/zcl_excel_writer_2007.clas.abap b/src/zcl_excel_writer_2007.clas.abap
index 26c7b6d..139c39c 100644
--- a/src/zcl_excel_writer_2007.clas.abap
+++ b/src/zcl_excel_writer_2007.clas.abap
@@ -3593,8 +3593,8 @@ CLASS zcl_excel_writer_2007 IMPLEMENTATION.
                                                        parent = lo_document ).
       lo_sub_element = lo_document->create_simple_element( name   = lc_xml_node_t
                                                            parent = lo_document ).
-      IF contains( val = <fs_sheet_string>-string_value start = ` ` ) = abap_true
-            OR contains( val = <fs_sheet_string>-string_value end = ` ` ) = abap_true.
+      IF boolc( contains( val = <fs_sheet_string>-string_value start = ` ` ) ) = abap_true
+            OR boolc( contains( val = <fs_sheet_string>-string_value end = ` ` ) ) = abap_true.
         lo_sub_element->set_attribute( name = 'space' namespace = 'xml' value = 'preserve' ).
       ENDIF.
       lo_sub_element->set_value( value = <fs_sheet_string>-string_value ).