diff --git a/src/vim9execute.c b/src/vim9execute.c
index de12d8e..dde95b5 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -4844,6 +4844,20 @@
 		int arg_set = tv->v_type != VAR_UNKNOWN
 				&& !(tv->v_type == VAR_SPECIAL
 					    && tv->vval.v_number == VVAL_NONE);
+
+		if (iptr->isn_type == ISN_JUMP_IF_ARG_NOT_SET && !arg_set)
+		{
+		    dfunc_T *df = ((dfunc_T *)def_functions.ga_data)
+							+ ectx->ec_dfunc_idx;
+		    ufunc_T *ufunc = df->df_ufunc;
+		    // jump_arg_off is negative for arguments
+		    size_t argidx = ufunc->uf_def_args.ga_len
+					+ iptr->isn_arg.jumparg.jump_arg_off
+					+ STACK_FRAME_SIZE;
+		    type_T *t = ufunc->uf_arg_types[argidx];
+		    tv->v_type = t->tt_type;
+		}
+
 		if (iptr->isn_type == ISN_JUMP_IF_ARG_SET ? arg_set : !arg_set)
 		    ectx->ec_iidx = iptr->isn_arg.jumparg.jump_where;
 		break;
@@ -5718,6 +5732,17 @@
 
 		    // The members are located right after the object struct.
 		    typval_T *mtv = ((typval_T *)(obj + 1)) + idx;
+		    if (mtv->v_type == VAR_UNKNOWN)
+		    {
+			// Referencing an object variable (without a type)
+			// which is not yet initialized.  So the type is not
+			// yet known.
+			ocmember_T *m = &obj->obj_class->class_obj_members[idx];
+			SOURCING_LNUM = iptr->isn_lnum;
+			semsg(_(e_uninitialized_object_var_reference),
+				m->ocm_name);
+			goto on_error;
+		    }
 		    copy_tv(mtv, tv);
 
 		    // Unreference the object after getting the member, it may