diff --git a/cpp.mk b/cpp.mk
index 4b33d9bc..bebbdee0 100644
--- a/cpp.mk
+++ b/cpp.mk
@@ -40,8 +40,6 @@ LIB_LINK_FLAGS=$(FLAGS_64) $(WARNING_FLAGS) -mcmodel=small -fPIC -Wl,-gc-section
 PROGRAM_FLAGS=$(FLAGS_64) $(WARNING_FLAGS) -I../../tlib/include/ -I../../printf/include/  -static -L../../tlib/ -ltlib -mcmodel=small -fPIC
 PROGRAM_LINK_FLAGS=$(FLAGS_64) $(WARNING_FLAGS) $(COMMON_LINK_FLAGS) -static -L../../tlib/ -ltlib -mcmodel=small -fPIC -z max-page-size=0x1000 -T ../linker.ld -Wl,-gc-sections
 
-D_FILES=
-
 # Generate the rules for the CPP files of a directory
 define compile_cpp_folder
 
@@ -51,12 +49,12 @@ $(1)/%.cpp.d: $(1)/%.cpp
 $(1)/%.cpp.o: $(1)/%.cpp
 	$(CXX) $(KERNEL_CPP_FLAGS_64) $(THOR_FLAGS) $(WARNING_FLAGS) -c $$< -o $(1)/$$*.cpp.o
 
-folder_cpp_files=$(wildcard $(1)/*.cpp)
-folder_d_files=$(folder_cpp_files:%.cpp=%.cpp.d)
-folder_o_files=$(folder_cpp_files:%.cpp=%.cpp.o)
+folder_cpp_files := $(wildcard $(1)/*.cpp)
+folder_d_files   := $(folder_cpp_files:%.cpp=%.cpp.d)
+folder_o_files   := $(folder_cpp_files:%.cpp=%.cpp.o)
 
-D_FILES += $(folder_d_files)
-O_FILES += $(folder_o_files)
+D_FILES := $(D_FILES) $(folder_d_files)
+O_FILES := $(O_FILES) $(folder_o_files)
 
 endef
 
@@ -69,11 +67,11 @@ acpica/source/components/$(1)/%.c.d: acpica/source/components/$(1)/%.c
 acpica/source/components/$(1)/%.c.o: acpica/source/components/$(1)/%.c
 	$(CC) $(ACPICA_C_FLAGS) $(THOR_FLAGS) -c $$< -o $$@
 
-folder_c_files=$(wildcard acpica/source/components/$(1)/*.c)
-folder_d_files=$(folder_c_files:%.c=%.c.d)
-folder_o_files=$(folder_c_files:%.c=%.c.o)
+acpica_folder_c_files := $(wildcard acpica/source/components/$(1)/*.c)
+acpica_folder_d_files := $$(acpica_folder_c_files:%.c=%.c.d)
+acpica_folder_o_files := $$(acpica_folder_c_files:%.c=%.c.o)
 
-D_FILES += $(folder_d_files)
-O_FILES += $(folder_o_files)
+D_FILES += $$(acpica_folder_d_files)
+O_FILES += $$(acpica_folder_o_files)
 
 endef