diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 75d64d344..1231f49ba 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -375,6 +375,9 @@ endif()
 
 check_include_files(endian.h HAVE_ENDIAN_H)
 
+if((NOT PROJECT_SOURCE_DIR STREQUAL PROJECT_BINARY_DIR) AND EXISTS "${PROJECT_SOURCE_DIR}/cmake_config.h")
+	message(FATAL_ERROR "You are doing an out-of-tree build, but build artifacts are left in-tree. This will break the build!")
+endif()
 configure_file(
 	"${PROJECT_SOURCE_DIR}/cmake_config.h.in"
 	"${PROJECT_BINARY_DIR}/cmake_config.h"