diff --git a/tmk_core/tool/mbed/gcc.mk b/tmk_core/tool/mbed/gcc.mk
index b8c7336b..596fbc8a 100644
--- a/tmk_core/tool/mbed/gcc.mk
+++ b/tmk_core/tool/mbed/gcc.mk
@@ -23,6 +23,7 @@ CC_FLAGS += \
 	-fdata-sections \
 	-fomit-frame-pointer
 CC_FLAGS += -MMD -MP
+CC_FLAGS += $(OPT_DEFS)
 
 LD_FLAGS = $(CPU) -Wl,--gc-sections --specs=nano.specs
 #LD_FLAGS += -u _printf_float -u _scanf_float