diff --git a/bare-arm/Makefile b/bare-arm/Makefile
index a59bb9f89a..dc17eed4d9 100644
--- a/bare-arm/Makefile
+++ b/bare-arm/Makefile
@@ -8,7 +8,7 @@ include ../py/py.mk
 
 CROSS_COMPILE = arm-none-eabi-
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I$(BUILD)
 
diff --git a/esp8266/Makefile b/esp8266/Makefile
index 7019335065..b06f6787a3 100644
--- a/esp8266/Makefile
+++ b/esp8266/Makefile
@@ -14,7 +14,7 @@ BAUD = 115200
 CROSS_COMPILE = xtensa-lx106-elf-
 ESP_SDK = $(shell $(CC) -print-sysroot)/usr
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I../stmhal
 INC += -I../lib/mp-readline
diff --git a/minimal/Makefile b/minimal/Makefile
index 09adfd4fd3..ddddebbe2e 100644
--- a/minimal/Makefile
+++ b/minimal/Makefile
@@ -12,7 +12,7 @@ ifeq ($(CROSS), 1)
 CROSS_COMPILE = arm-none-eabi-
 endif
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I../lib/mp-readline
 INC += -I../stmhal
diff --git a/pic16bit/Makefile b/pic16bit/Makefile
index 4fab7f61c2..98fcf07fd9 100644
--- a/pic16bit/Makefile
+++ b/pic16bit/Makefile
@@ -12,7 +12,7 @@ CROSS_COMPILE = $(XC16)/bin/xc16-
 PARTFAMILY = dsPIC33F
 PART = 33FJ256GP506
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I../lib/mp-readline
 INC += -I../stmhal
diff --git a/py/py.mk b/py/py.mk
index 12773ff8fa..b3a991936d 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -10,6 +10,8 @@ PY_QSTR_DEFS = $(PY_SRC)/qstrdefs.h
 # some code is performance bottleneck and compiled with other optimization options
 CSUPEROPT = -O3
 
+INC += -I../lib/netutils
+
 ifeq ($(MICROPY_PY_USSL),1)
 CFLAGS_MOD += -DMICROPY_PY_USSL=1 -I../lib/axtls/ssl -I../lib/axtls/crypto -I../lib/axtls/config
 LDFLAGS_MOD += -L../lib/axtls/_stage -laxtls
diff --git a/qemu-arm/Makefile b/qemu-arm/Makefile
index 0c09dd2ffe..a81dd67cf1 100644
--- a/qemu-arm/Makefile
+++ b/qemu-arm/Makefile
@@ -9,7 +9,7 @@ include ../py/py.mk
 
 CROSS_COMPILE = arm-none-eabi-
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I$(BUILD)
 INC += -I../tools/tinytest/
diff --git a/stmhal/Makefile b/stmhal/Makefile
index ce5ab639d8..5695fe584e 100644
--- a/stmhal/Makefile
+++ b/stmhal/Makefile
@@ -32,7 +32,7 @@ DEVICE=0483:df11
 
 CROSS_COMPILE = arm-none-eabi-
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I$(BUILD)
 INC += -I$(CMSIS_DIR)/inc
diff --git a/teensy/Makefile b/teensy/Makefile
index 4e97cbc6a9..dece6ce003 100644
--- a/teensy/Makefile
+++ b/teensy/Makefile
@@ -29,7 +29,7 @@ endif
 CFLAGS_TEENSY = -DF_CPU=96000000 -DUSB_SERIAL -D__MK20DX256__
 CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mcpu=cortex-m4 -msoft-float -mfloat-abi=soft -fsingle-precision-constant -Wdouble-promotion $(CFLAGS_TEENSY)
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I../stmhal
 INC += -I../lib/mp-readline
diff --git a/unix/Makefile b/unix/Makefile
index bd79855554..2a27364004 100644
--- a/unix/Makefile
+++ b/unix/Makefile
@@ -13,7 +13,7 @@ UNAME_S := $(shell uname -s)
 # include py core make definitions
 include ../py/py.mk
 
-INC =  -I.
+INC +=  -I.
 INC +=  -I..
 INC += -I$(BUILD)
 
diff --git a/windows/Makefile b/windows/Makefile
index e3a2fac422..d791735596 100644
--- a/windows/Makefile
+++ b/windows/Makefile
@@ -10,7 +10,7 @@ QSTR_DEFS = ../unix/qstrdefsport.h
 # include py core make definitions
 include ../py/py.mk
 
-INC =  -I.
+INC += -I.
 INC += -I..
 INC += -I$(BUILD)