diff --git a/Modules/Compiler/IAR-CXX.cmake b/Modules/Compiler/IAR-CXX.cmake
index 6c15735e7f..3f0ef1f357 100644
--- a/Modules/Compiler/IAR-CXX.cmake
+++ b/Modules/Compiler/IAR-CXX.cmake
@@ -22,9 +22,9 @@ if(NOT CMAKE_IAR_CXX_FLAG)
   set(_CMAKE_IAR_MODERNCXX_LIST 14 17)
   if(${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT} IN_LIST _CMAKE_IAR_MODERNCXX_LIST OR
      ("${CMAKE_CXX_COMPILER_ARCHITECTURE_ID}" STREQUAL "ARM" AND ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT} EQUAL 98))
-    string(PREPEND CMAKE_CXX_FLAGS "--c++ ")
+    set(CMAKE_IAR_CXX_FLAG --c++)
   else()
-    string(PREPEND CMAKE_CXX_FLAGS "--eec++ ")
+    set(CMAKE_IAR_CXX_FLAG --eec++)
   endif()
   unset(_CMAKE_IAR_MODERNCXX_LIST)