diff --git a/xs/lib/Slic3r/XS.pm b/xs/lib/Slic3r/XS.pm
index 5cac751b4..9f761ba9a 100644
--- a/xs/lib/Slic3r/XS.pm
+++ b/xs/lib/Slic3r/XS.pm
@@ -4,6 +4,7 @@ use strict;
 
 our $VERSION = '0.01';
 
+use Carp qw();
 use XSLoader;
 XSLoader::load(__PACKAGE__, $VERSION);
 
diff --git a/xs/src/TriangleMesh.cpp b/xs/src/TriangleMesh.cpp
index b1abae55b..664c7c26d 100644
--- a/xs/src/TriangleMesh.cpp
+++ b/xs/src/TriangleMesh.cpp
@@ -11,7 +11,6 @@
 
 namespace Slic3r {
 
-TriangleMesh::TriangleMesh() {}
 TriangleMesh::~TriangleMesh() {
     stl_close(&stl);
 }
@@ -112,6 +111,8 @@ TriangleMesh::Repair() {
     
     // neighbors
     stl_verify_neighbors(&stl);
+    
+    this->repaired = true;
 }
 
 void
@@ -484,6 +485,9 @@ TriangleMesh::split() const
     TriangleMeshPtrs meshes;
     std::set<int> seen_facets;
     
+    // we need neighbors
+    if (!this->repaired) CONFESS("split() requires Repair()");
+    
     // loop while we have remaining facets
     while (1) {
         // get the first facet
@@ -530,8 +534,8 @@ TriangleMesh::merge(const TriangleMesh* mesh)
 {
     // reset stats and metadata
     int number_of_facets = this->stl.stats.number_of_facets;
-    stl_initialize(&this->stl);
     stl_invalidate_shared_vertices(&this->stl);
+    this->repaired = false;
     
     // update facet count and allocate more memory
     this->stl.stats.number_of_facets = number_of_facets + mesh->stl.stats.number_of_facets;
@@ -542,6 +546,9 @@ TriangleMesh::merge(const TriangleMesh* mesh)
     for (int i = 0; i < mesh->stl.stats.number_of_facets; i++) {
         this->stl.facet_start[number_of_facets + i] = mesh->stl.facet_start[i];
     }
+    
+    // update size
+    stl_get_size(&this->stl);
 }
 
 }
diff --git a/xs/src/TriangleMesh.hpp b/xs/src/TriangleMesh.hpp
index 77ef6a4d4..e6a1a8a08 100644
--- a/xs/src/TriangleMesh.hpp
+++ b/xs/src/TriangleMesh.hpp
@@ -15,7 +15,7 @@ typedef std::vector<TriangleMesh*> TriangleMeshPtrs;
 class TriangleMesh
 {
     public:
-    TriangleMesh();
+    TriangleMesh() : repaired(false) {};
     ~TriangleMesh();
     SV* to_SV();
     void ReadSTLFile(char* input_file);
@@ -30,6 +30,7 @@ class TriangleMesh
     TriangleMeshPtrs split() const;
     void merge(const TriangleMesh* mesh);
     stl_file stl;
+    bool repaired;
 };
 
 enum FacetEdgeType { feNone, feTop, feBottom };
diff --git a/xs/t/01_trianglemesh.t b/xs/t/01_trianglemesh.t
index 6c4b062e1..24be62fbc 100644
--- a/xs/t/01_trianglemesh.t
+++ b/xs/t/01_trianglemesh.t
@@ -4,7 +4,7 @@ use strict;
 use warnings;
 
 use Slic3r::XS;
-use Test::More tests => 46;
+use Test::More tests => 47;
 
 is Slic3r::TriangleMesh::XS::hello_world(), 'Hello world!',
     'hello world';
@@ -43,14 +43,23 @@ my $cube = {
     $m->rotate(45, Slic3r::Point->new(20,20));
     ok abs($m->size->[0] - sqrt(2)*40) < 1E-4, 'rotate';
     
-    my $result = $m->split;
-    is scalar(@$result), 1, 'split';
-    isa_ok $result->[0], 'Slic3r::TriangleMesh::XS', 'split';
+    {
+        my $meshes = $m->split;
+        is scalar(@$meshes), 1, 'split';
+        isa_ok $meshes->[0], 'Slic3r::TriangleMesh::XS', 'split';
+    }
     
     my $m2 = Slic3r::TriangleMesh::XS->new;
     $m2->ReadFromPerl($cube->{vertices}, $cube->{facets});
+    $m2->Repair;
     $m->merge($m2);
+    $m->Repair;
     is $m->stats->{number_of_facets}, 2 * $m2->stats->{number_of_facets}, 'merge';
+    
+    {
+        my $meshes = $m->split;
+        is scalar(@$meshes), 2, 'split';
+    }
 }
 
 {
diff --git a/xs/xsp/TriangleMesh.xsp b/xs/xsp/TriangleMesh.xsp
index 02f2f637d..433e63508 100644
--- a/xs/xsp/TriangleMesh.xsp
+++ b/xs/xsp/TriangleMesh.xsp
@@ -41,6 +41,8 @@ TriangleMesh::stats()
 SV*
 TriangleMesh::vertices()
     CODE:
+        if (!THIS->repaired) CONFESS("vertices() requires Repair()");
+        
         if (THIS->stl.v_shared == NULL)
             stl_generate_shared_vertices(&(THIS->stl));
         
@@ -63,6 +65,8 @@ TriangleMesh::vertices()
 SV*
 TriangleMesh::facets()
     CODE:
+        if (!THIS->repaired) CONFESS("facets() requires Repair()");
+        
         if (THIS->stl.v_shared == NULL)
             stl_generate_shared_vertices(&(THIS->stl));