diff --git a/src/cpu/checker/cpu.hh b/src/cpu/checker/cpu.hh
index b5fdc64..c455cf5 100644
--- a/src/cpu/checker/cpu.hh
+++ b/src/cpu/checker/cpu.hh
@@ -176,7 +176,10 @@
     RegVal
     getRegOperand(const StaticInst *si, int idx) override
     {
-        return thread->getReg(si->srcRegIdx(idx));
+        const RegId& id = si->srcRegIdx(idx);
+        if (id.is(InvalidRegClass))
+            return 0;
+        return thread->getReg(id);
     }
 
     void