diff --git a/ports/stm32f4/peripherals/stm32f4/stm32f405xx/periph.h b/ports/stm32f4/peripherals/stm32f4/stm32f405xx/periph.h
index c755623982..e87e798574 100644
--- a/ports/stm32f4/peripherals/stm32f4/stm32f405xx/periph.h
+++ b/ports/stm32f4/peripherals/stm32f4/stm32f405xx/periph.h
@@ -43,7 +43,7 @@ extern const mcu_spi_nss_obj_t mcu_spi_nss_list[6];
 
 //UART
 extern USART_TypeDef * mcu_uart_banks[MAX_UART];
-bool mcu_uart_has_usart[MAX_UART];
+extern bool mcu_uart_has_usart[MAX_UART];
 
 extern const mcu_uart_tx_obj_t mcu_uart_tx_list[12];
 extern const mcu_uart_rx_obj_t mcu_uart_rx_list[12];
diff --git a/ports/stm32f4/peripherals/stm32f4/stm32f411xe/periph.h b/ports/stm32f4/peripherals/stm32f4/stm32f411xe/periph.h
index dda68f40ec..d657d73b68 100644
--- a/ports/stm32f4/peripherals/stm32f4/stm32f411xe/periph.h
+++ b/ports/stm32f4/peripherals/stm32f4/stm32f411xe/periph.h
@@ -43,7 +43,7 @@ extern const mcu_spi_nss_obj_t mcu_spi_nss_list[12];
 
 //UART
 extern USART_TypeDef * mcu_uart_banks[MAX_UART];
-bool mcu_uart_has_usart[MAX_UART];
+extern bool mcu_uart_has_usart[MAX_UART];
 
 extern const mcu_uart_tx_obj_t mcu_uart_tx_list[7];
 extern const mcu_uart_rx_obj_t mcu_uart_rx_list[7];
diff --git a/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.c b/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.c
index 19ef806dda..f5effa80ed 100644
--- a/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.c
+++ b/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.c
@@ -122,8 +122,8 @@ const mcu_spi_nss_obj_t mcu_spi_nss_list[12] = {
 
 //UART
 
-USART_TypeDef * mcu_uart_banks[6] = {USART1, USART2, USART3, NULL, NULL, USART6};
-bool mcu_uart_has_usart[6] = {true, true, true, false, false, true};
+USART_TypeDef * mcu_uart_banks[MAX_UART] = {USART1, USART2, USART3, NULL, NULL, USART6};
+bool mcu_uart_has_usart[MAX_UART] = {true, true, true, false, false, true};
 
 const mcu_uart_tx_obj_t mcu_uart_tx_list[11] = {
     UART(2, 7, &pin_PA02),
diff --git a/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.h b/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.h
index 7a3eaa66ef..e0141e6dab 100644
--- a/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.h
+++ b/ports/stm32f4/peripherals/stm32f4/stm32f412zx/periph.h
@@ -44,7 +44,7 @@ extern const mcu_spi_nss_obj_t mcu_spi_nss_list[12];
 
 //UART
 extern USART_TypeDef * mcu_uart_banks[MAX_UART];
-bool mcu_uart_has_usart[MAX_UART];
+extern bool mcu_uart_has_usart[MAX_UART];
 
 extern const mcu_uart_tx_obj_t mcu_uart_tx_list[11];
 extern const mcu_uart_rx_obj_t mcu_uart_rx_list[12];