diff --git a/CMakeLists.txt b/CMakeLists.txt
index ea56014f44b8b1c66c295a2f0fa409bbe0f30d3d..eaf02bea2fbce4de6138e74c83253a2c9ff131da 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -64,5 +64,5 @@ target_link_libraries(xpdevplus ciolib comio encode hash uifc xpdev)
 target_include_directories(xpdevplus PUBLIC ${XPDevPlus_BINARY_DIR})
 target_compile_options(xpdevplus PUBLIC "SHELL:-include xpdevplus.h")
 if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
-target_compile_definitions(xpdevplus PUBLIC POSIX_C_SOURCE 200809L _DEFAULT_SOURCE _BSD_SOURCE SPEED_MACROS_ONLY _GNU_SOURCE _FILE_OFFSET_BITS=64)
+target_compile_definitions(xpdevplus PUBLIC POSIX_C_SOURCE=200809L _DEFAULT_SOURCE _BSD_SOURCE SPEED_MACROS_ONLY _GNU_SOURCE _FILE_OFFSET_BITS=64)
 endif()