diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index f5a61f25..b1276754 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -1,6 +1,7 @@
 # flex
 find_package(FLEX REQUIRED)
 flex_target(scan src/scan.l ${PROJECT_SOURCE_DIR}/src/lex.yy.c COMPILE_FLAGS --header-file="src/lex.yy.h")
+set_directory_properties(PROPERTIES ADDITIONAL_MAKE_CLEAN_FILES ${PROJECT_SOURCE_DIR}/src/lex.yy.h)
 
 # build!
 file(GLOB C_SOURCES ${PROJECT_SOURCE_DIR}/src/*.c)