diff --git a/runtime.c b/runtime.c
index 9bbfe4b8..ed238a24 100644
--- a/runtime.c
+++ b/runtime.c
@@ -2833,6 +2833,7 @@ char *gc_move(char *obj, gc_thread_data *thd, int *alloci, int *heap_grown) {
     }
     case forward_tag:
       return (char *)forward(obj);
+    case c_opaque_tag: break;
     case eof_tag: break;
     case primitive_tag: break;
     case boolean_tag: break;
@@ -2966,6 +2967,7 @@ int gc_minor(void *data, object low_limit, object high_limit, closure cont, obje
       case cvar_tag:
         break;
       // These types are not heap-allocated
+      case c_opaque_tag:
       case eof_tag:
       case primitive_tag:
       case symbol_tag: