diff --git a/cpu/stm32f3/periph/spi.c b/cpu/stm32f3/periph/spi.c
index 6a0bb22472..e8c045e16d 100644
--- a/cpu/stm32f3/periph/spi.c
+++ b/cpu/stm32f3/periph/spi.c
@@ -290,7 +290,7 @@ int spi_conf_pins(spi_t dev)
 
 int spi_acquire(spi_t dev)
 {
-    if (dev >= SPI_NUMOF) {
+    if ((unsigned int)dev >= SPI_NUMOF) {
         return -1;
     }
     mutex_lock(&locks[dev]);
@@ -299,7 +299,7 @@ int spi_acquire(spi_t dev)
 
 int spi_release(spi_t dev)
 {
-    if (dev >= SPI_NUMOF) {
+    if ((unsigned int)dev >= SPI_NUMOF) {
         return -1;
     }
     mutex_unlock(&locks[dev]);
@@ -335,7 +335,7 @@ int spi_transfer_byte(spi_t dev, char out, char *in)
 
 void spi_transmission_begin(spi_t dev, char reset_val)
 {
-    if (dev < SPI_NUMOF) {
+    if ((unsigned int)dev < SPI_NUMOF) {
         spi[dev]->DR = reset_val;
     }
 }