diff --git a/target/avr/helper.c b/target/avr/helper.c
index 2bad242a66..e6e7d51487 100644
--- a/target/avr/helper.c
+++ b/target/avr/helper.c
@@ -52,7 +52,7 @@ bool avr_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
     }
     if (interrupt_request & CPU_INTERRUPT_HARD) {
         if (cpu_interrupts_enabled(env) && env->intsrc != 0) {
-            int index = ctz32(env->intsrc);
+            int index = ctz64(env->intsrc);
             cs->exception_index = EXCP_INT(index);
             avr_cpu_do_interrupt(cs);
 
@@ -79,7 +79,7 @@ void avr_cpu_do_interrupt(CPUState *cs)
     if (cs->exception_index == EXCP_RESET) {
         vector = 0;
     } else if (env->intsrc != 0) {
-        vector = ctz32(env->intsrc) + 1;
+        vector = ctz64(env->intsrc) + 1;
     }
 
     if (avr_feature(env, AVR_FEATURE_3_BYTE_PC)) {