diff --git a/src/expr.cc b/src/expr.cc
index 0769d575..5bc537d9 100644
--- a/src/expr.cc
+++ b/src/expr.cc
@@ -52,8 +52,9 @@ void expr_t::parse(std::istream& in, const parse_flags_t& flags,
     in.seekg(start_pos, std::ios::beg);
     scoped_array<char> buf
       (new char[static_cast<std::size_t>(end_pos - start_pos) + 1]);
-    in.read(buf.get(), end_pos - start_pos);
-    buf[end_pos - start_pos] = '\0';
+    int len = static_cast<int>(end_pos) - static_cast<int>(start_pos);
+    in.read(buf.get(), len);
+    buf[len] = '\0';
     set_text(buf.get());
   }
   else {