diff --git a/ports/stm32/lwip_inc/arch/cc.h b/ports/stm32/lwip_inc/arch/cc.h
index 635b1c8056..fc5230ef72 100644
--- a/ports/stm32/lwip_inc/arch/cc.h
+++ b/ports/stm32/lwip_inc/arch/cc.h
@@ -5,4 +5,6 @@
 #define LWIP_PLATFORM_DIAG(x)
 #define LWIP_PLATFORM_ASSERT(x)  { assert(1); }
 
+#define LWIP_NO_CTYPE_H 1
+
 #endif // MICROPY_INCLUDED_STM32_LWIP_ARCH_CC_H