diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 32703b257..79701ac0e 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -1763,7 +1763,7 @@ function(_ep_add_configure_command name)
     if(cmake_generator)
       list(APPEND cmd "-G${cmake_generator}")
       if(cmake_generator_platform)
-        list(APPEND cmd "-DCMAKE_GENERATOR_PLATFORM=${cmake_generator_platform}")
+        list(APPEND cmd "-A${cmake_generator_platform}")
       endif()
       if(cmake_generator_toolset)
         list(APPEND cmd "-T${cmake_generator_toolset}")
@@ -1778,7 +1778,7 @@ function(_ep_add_configure_command name)
         message(FATAL_ERROR "Option CMAKE_GENERATOR_PLATFORM not allowed without CMAKE_GENERATOR.")
       endif()
       if(CMAKE_GENERATOR_PLATFORM)
-        list(APPEND cmd "-DCMAKE_GENERATOR_PLATFORM=${CMAKE_GENERATOR_PLATFORM}")
+        list(APPEND cmd "-A${CMAKE_GENERATOR_PLATFORM}")
       endif()
       if(cmake_generator_toolset)
         message(FATAL_ERROR "Option CMAKE_GENERATOR_TOOLSET not allowed without CMAKE_GENERATOR.")