diff --git a/include/mapnik/mapped_memory_cache.hpp b/include/mapnik/mapped_memory_cache.hpp
index 6b6102c4e..c67d63ad7 100644
--- a/include/mapnik/mapped_memory_cache.hpp
+++ b/include/mapnik/mapped_memory_cache.hpp
@@ -52,6 +52,8 @@ public:
     void clear();
 };
 
+extern template class MAPNIK_DECL singleton<mapped_memory_cache, CreateStatic>;
+
 }
 
 #endif // MAPNIK_MAPPED_MEMORY_CACHE_HPP
diff --git a/src/mapped_memory_cache.cpp b/src/mapped_memory_cache.cpp
index db4fce897..f35a379af 100644
--- a/src/mapped_memory_cache.cpp
+++ b/src/mapped_memory_cache.cpp
@@ -37,6 +37,8 @@
 namespace mapnik
 {
 
+template class singleton<mapped_memory_cache, CreateStatic>;
+
 void mapped_memory_cache::clear()
 {
 #ifdef MAPNIK_THREADSAFE
@@ -58,6 +60,7 @@ boost::optional<mapped_region_ptr> mapped_memory_cache::find(std::string const&
 #ifdef MAPNIK_THREADSAFE
     std::lock_guard<std::mutex> lock(mutex_);
 #endif
+
     using iterator_type = std::unordered_map<std::string, mapped_region_ptr>::const_iterator;
     boost::optional<mapped_region_ptr> result;
     iterator_type itr = cache_.find(uri);
@@ -76,7 +79,7 @@ boost::optional<mapped_region_ptr> mapped_memory_cache::find(std::string const&
             result.reset(region);
             if (update_cache)
             {
-                cache_.emplace(uri,*result);
+                cache_.emplace(uri, *result);
             }
             return result;
         }
diff --git a/test/unit/datasource/shapeindex.cpp b/test/unit/datasource/shapeindex.cpp
index a0eef2d02..51f47ac7c 100644
--- a/test/unit/datasource/shapeindex.cpp
+++ b/test/unit/datasource/shapeindex.cpp
@@ -24,6 +24,7 @@
 
 #include <mapnik/datasource.hpp>
 #include <mapnik/datasource_cache.hpp>
+#include <mapnik/mapped_memory_cache.hpp>
 #include <mapnik/util/fs.hpp>
 #include <cstdlib>
 #include <fstream>
@@ -36,6 +37,9 @@ namespace {
 
 std::size_t count_shapefile_features(std::string const& filename)
 {
+#if defined(MAPNIK_MEMORY_MAPPED_FILE)
+    mapnik::mapped_memory_cache::instance().clear();
+#endif
     mapnik::parameters params;
     params["type"] = "shape";
     params["file"] = filename;
@@ -93,31 +97,41 @@ TEST_CASE("invalid shapeindex")
     {
         SECTION("Invalid index")
         {
-            std::string path = "test/data/shp/boundaries.shp";
-            std::string index_path = path.substr(0, path.rfind(".")) + ".index";
-            // remove *.index if present
-            if (mapnik::util::exists(index_path))
+            for (auto val : {std::make_tuple(true, std::string("mapnik-invalid-index.................")), // invalid header
+                           std::make_tuple(false, std::string("mapnik-index................."))})       // valid header + invalid index
             {
-                mapnik::util::remove(index_path);
-            }
-            // count features
+                std::string path = "test/data/shp/boundaries.shp";
+                std::string index_path = path.substr(0, path.rfind(".")) + ".index";
+                // remove *.index if present
+                if (mapnik::util::exists(index_path))
+                {
+                    mapnik::util::remove(index_path);
+                }
+                // count features
 
-            std::size_t feature_count = count_shapefile_features(path);
+                std::size_t feature_count = count_shapefile_features(path);
 
-            // create invalid index
-            std::ofstream index(index_path.c_str(), std::ios::binary);
-            std::string buf("mapnik-invalid-index.................");
-            index.write(buf.c_str(), buf.size());
-            index.close();
+                // create index
+                std::ofstream index(index_path.c_str(), std::ios::binary);
+                index.write(std::get<1>(val).c_str(), std::get<1>(val).size());
+                index.close();
 
-            // count features
-            std::size_t feature_count_indexed = count_shapefile_features(path);
-            // ensure number of features are the same
-            REQUIRE(feature_count == feature_count_indexed);
-            // remove *.index if present
-            if (mapnik::util::exists(index_path))
-            {
-                mapnik::util::remove(index_path);
+                // count features
+                std::size_t feature_count_indexed = count_shapefile_features(path);
+                if (std::get<0>(val)) // fallback to un-indexed access
+                {
+                    // ensure number of features are the same
+                    CHECK(feature_count == feature_count_indexed);
+                }
+                else // the header is valid but index file itself is not - expect datasource to fail and return 0 features.
+                {
+                    CHECK(feature_count_indexed == 0);
+                }
+                // remove *.index if present
+                if (mapnik::util::exists(index_path))
+                {
+                    mapnik::util::remove(index_path);
+                }
             }
         }
     }