diff --git a/stage0/sema.c b/stage0/sema.c
index 7651e0f8d2..88fcf62000 100644
--- a/stage0/sema.c
+++ b/stage0/sema.c
@@ -2471,9 +2471,18 @@ static AirInstRef zirRem(Sema* sema, SemaBlock* block, uint32_t inst) {
 // zirModRem: handle plain '%' operator (may be mod or rem depending on types).
 // Ported from src/Sema.zig zirModRem.
 static AirInstRef zirModRem(Sema* sema, SemaBlock* block, uint32_t inst) {
-    // Ported from Sema.zig zirModRem: for unsigned types, same as @rem.
-    // Signed types would need distinct handling (mod vs rem ambiguity).
-    // TODO: check signedness and UNIMPLEMENTED for signed types.
+    // Ported from Sema.zig zirModRem: for unsigned integers, '%' is
+    // equivalent to @rem. For signed integers, '%' is @mod which
+    // needs distinct handling (not yet ported).
+    uint32_t payload_index = sema->code.inst_datas[inst].pl_node.payload_index;
+    AirInstRef lhs = resolveInst(sema, sema->code.extra[payload_index]);
+    TypeIndex lhs_ty = semaTypeOf(sema, lhs);
+    if (lhs_ty != IP_INDEX_COMPTIME_INT_TYPE
+        && sema->ip->items[lhs_ty].tag == IP_KEY_INT_TYPE) {
+        InternPoolKey k = ipIndexToKey(sema->ip, lhs_ty);
+        if (k.data.int_type.signedness == 1)
+            UNIMPLEMENTED("zirModRem: signed integer mod");
+    }
     return zirRem(sema, block, inst);
 }
 
@@ -11324,8 +11333,8 @@ static AirInstRef zirExtended(Sema* sema, SemaBlock* block, uint32_t inst) {
                 }
             }
         }
-        // TODO: implement remaining builtin_value kinds
-        return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE);
+        UNIMPLEMENTED("zirBuiltinValue: unimplemented builtin_value kind");
+        return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE); // unreachable
     }
     UNIMPLEMENTED("zirExtended: unimplemented extended opcode");
     return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE); // unreachable