diff --git a/CMakeLists.txt b/CMakeLists.txt
index 690220714..7b95a4e16 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
+if(${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
   message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the SDL source code and call cmake from there")
 endif()