diff --git a/Arduino.mk b/Arduino.mk
index 5b0af0f..4646dad 100644
--- a/Arduino.mk
+++ b/Arduino.mk
@@ -1014,7 +1014,7 @@ endif
 
 # Using += instead of =, so that CPPFLAGS can be set per sketch level
 CPPFLAGS      += -$(MCU_FLAG_NAME)=$(MCU) -DF_CPU=$(F_CPU) -DARDUINO=$(ARDUINO_VERSION) $(ARDUINO_ARCH_FLAG) -D__PROG_TYPES_COMPAT__ \
-        -I$(ARDUINO_CORE_PATH) -I$(ARDUINO_VAR_PATH)/$(VARIANT) \
+        -I$(ARDUINO_CORE_PATH) -I$(ARDUINO_SKETCHBOOK) -I$(ARDUINO_VAR_PATH)/$(VARIANT) \
         $(SYS_INCLUDES) $(PLATFORM_INCLUDES) $(USER_INCLUDES) -Wall -ffunction-sections \
         -fdata-sections