diff --git a/cpu/cc2538/periph/adc.c b/cpu/cc2538/periph/adc.c
index 3b8a705389..07ab6fc3ad 100644
--- a/cpu/cc2538/periph/adc.c
+++ b/cpu/cc2538/periph/adc.c
@@ -51,7 +51,7 @@ int adc_init(adc_t line)
     return 0;
 }
 
-int adc_sample(adc_t line, adc_res_t res)
+int32_t adc_sample(adc_t line, adc_res_t res)
 {
     /* check if adc line valid */
     if (line >= ADC_NUMOF) {