diff --git a/cpu/stm32f0/periph/spi.c b/cpu/stm32f0/periph/spi.c
index 6a337c97d2..3b35557dd3 100644
--- a/cpu/stm32f0/periph/spi.c
+++ b/cpu/stm32f0/periph/spi.c
@@ -162,7 +162,7 @@ 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]);
@@ -171,7 +171,7 @@ 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]);