diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0752665..0297a56 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -62,7 +62,7 @@ endif()
 
 # Define __CMT_FILENAME__ consistently across Operating Systems
 if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Windows")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__CMT_FILENAME__='\"$(subst ${CMAKE_SOURCE_DIR}/,,$(abspath $<))\"'")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__CMT_FILENAME__='\"$$(subst ${CMAKE_SOURCE_DIR}/,,$$(abspath $$<))\"'")
 else()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__CMT_FILENAME__=__FILE__")
 endif()