diff --git a/cpu/sam0_common/periph/adc.c b/cpu/sam0_common/periph/adc.c
index db97790480..7760a2b935 100644
--- a/cpu/sam0_common/periph/adc.c
+++ b/cpu/sam0_common/periph/adc.c
@@ -82,12 +82,13 @@ static int _adc_configure(adc_res_t res)
     /* Individual comparison necessary because ADC Resolution Bits are not
      * numerically in order and 16Bit (averaging - not currently supported)
      * falls between 12bit and 10bit.  See datasheet for details */
-    assert((res == ADC_RES_8BIT) || (res == ADC_RES_10BIT) ||
-           (res == ADC_RES_12BIT));
+    if (!((res == ADC_RES_8BIT) || (res == ADC_RES_10BIT) ||
+          (res == ADC_RES_12BIT))){
+        return -1;
+    }
     _adc_poweroff();
     if (ADC->CTRLA.reg & ADC_CTRLA_SWRST ||
         ADC->CTRLA.reg & ADC_CTRLA_ENABLE ) {
-        _done();
         DEBUG("adc: not ready\n");
         return -1;
     }