diff --git a/include/mapnik/util/spatial_index.hpp b/include/mapnik/util/spatial_index.hpp
index da30f058e..b0c26ed4b 100644
--- a/include/mapnik/util/spatial_index.hpp
+++ b/include/mapnik/util/spatial_index.hpp
@@ -69,7 +69,7 @@ bool spatial_index<Value, Filter, InputStream>::check_header(InputStream& in)
 template <typename Value, typename Filter, typename InputStream>
 box2d<double> spatial_index<Value, Filter, InputStream>::bounding_box(InputStream& in)
 {
-    if (!check_header(in)) throw std::runtime_error("Invalid index file");
+    if (!check_header(in)) throw std::runtime_error("Invalid index file (regenerate with shapeindex)");
     in.seekg(16 + 4, std::ios::beg);
     box2d<double> box;
     read_envelope(in, box);
@@ -80,7 +80,7 @@ box2d<double> spatial_index<Value, Filter, InputStream>::bounding_box(InputStrea
 template <typename Value, typename Filter, typename InputStream>
 void spatial_index<Value, Filter, InputStream>::query(Filter const& filter, InputStream& in, std::vector<Value>& results)
 {
-    if (!check_header(in)) throw std::runtime_error("Invalid index file");
+    if (!check_header(in)) throw std::runtime_error("Invalid index file (regenerate with shapeindex)");
     in.seekg(16, std::ios::beg);
     query_node(filter, in, results);
 }
@@ -115,7 +115,7 @@ void spatial_index<Value, Filter, InputStream>::query_node(Filter const& filter,
 template <typename Value, typename Filter, typename InputStream>
 void spatial_index<Value, Filter, InputStream>::query_first_n(Filter const& filter, InputStream& in, std::vector<Value>& results, std::size_t count)
 {
-    if (!check_header(in)) throw std::runtime_error("Invalid index file");
+    if (!check_header(in)) throw std::runtime_error("Invalid index file (regenerate with shapeindex)");
     in.seekg(16, std::ios::beg);
     query_first_n_impl(filter, in, results, count);
 }