diff --git a/ports/stm32/lwip_inc/arch/cc.h b/ports/stm32/lwip_inc/arch/cc.h
index fc5230ef7..a818d6d6a 100644
--- a/ports/stm32/lwip_inc/arch/cc.h
+++ b/ports/stm32/lwip_inc/arch/cc.h
@@ -2,7 +2,9 @@
 #define MICROPY_INCLUDED_STM32_LWIP_ARCH_CC_H
 
 #include <assert.h>
-#define LWIP_PLATFORM_DIAG(x)
+#include <stdio.h>
+
+#define LWIP_PLATFORM_DIAG(x)    do { printf x; } while (0)
 #define LWIP_PLATFORM_ASSERT(x)  { assert(1); }
 
 #define LWIP_NO_CTYPE_H 1