diff --git a/arch/avr32/cpu/at32ap700x/clk.c b/arch/avr32/cpu/at32ap700x/clk.c
index d5dbe3b..0fc6088 100644
--- a/arch/avr32/cpu/at32ap700x/clk.c
+++ b/arch/avr32/cpu/at32ap700x/clk.c
@@ -72,7 +72,7 @@
 		sm_writel(PM_GCCTRL(id), parent | SM_BIT(CEN));
 		rate = parent_rate;
 	} else {
-		divider = min(255, divider / 2 - 1);
+		divider = min(255UL, divider / 2 - 1);
 		sm_writel(PM_GCCTRL(id), parent | SM_BIT(CEN) | SM_BIT(DIVEN)
 				| SM_BF(DIV, divider));
 		rate = parent_rate / (2 * (divider + 1));