diff --git a/CMakeLists.txt b/CMakeLists.txt
index 98ee88f..94054ce 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -42,7 +42,7 @@ endif()
 
 # Define __FILENAME__ consistently across Operating Systems
 if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Windows")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__='\"$(subst ${CMAKE_SOURCE_DIR}/,,$(abspath $<))\"'")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__='\"$$(subst ${CMAKE_SOURCE_DIR}/,,$$(abspath $$<))\"'")
 else()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__=__FILE__")
 endif()