diff --git a/src/runtime/c/pgf/expr.c b/src/runtime/c/pgf/expr.c
index 6f2de5a6b..ad8489808 100644
--- a/src/runtime/c/pgf/expr.c
+++ b/src/runtime/c/pgf/expr.c
@@ -350,6 +350,8 @@ pgf_print_expr(PgfExpr expr, int prec,
 		break;
 	}
 	case PGF_EXPR_META:
+		gu_putc('?', wtr, err);
+		break;
 	case PGF_EXPR_VAR:
 	case PGF_EXPR_TYPED:
 	case PGF_EXPR_IMPL_ARG: