diff --git a/lib/Slic3r/Format/AMF.pm b/lib/Slic3r/Format/AMF.pm
index ec6b31fe4..218b1c29d 100644
--- a/lib/Slic3r/Format/AMF.pm
+++ b/lib/Slic3r/Format/AMF.pm
@@ -36,7 +36,9 @@ sub write_file {
     printf $fh qq{<amf unit="millimeter">\n};
     printf $fh qq{  <metadata type="cad">Slic3r %s</metadata>\n}, $Slic3r::VERSION;
     for my $material_id (sort @{ $model->material_names }) {
+        next if $material_id eq '';
         my $material = $model->get_material($material_id);
+        # note that material-id must never be 0 since it's reserved by the AMF spec
         printf $fh qq{  <material id="%s">\n}, $material_id;
         for (keys %{$material->attributes}) {
              printf $fh qq{    <metadata type=\"%s\">%s</metadata>\n}, $_, $material->attributes->{$_};
diff --git a/xs/src/libslic3r/Model.cpp b/xs/src/libslic3r/Model.cpp
index 8d98b4d6b..29a2f3310 100644
--- a/xs/src/libslic3r/Model.cpp
+++ b/xs/src/libslic3r/Model.cpp
@@ -690,7 +690,8 @@ ModelVolume::assign_unique_material()
 {
     Model* model = this->get_object()->get_model();
     
-    this->_material_id = 1 + model->materials.size();
+    // as material-id "0" is reserved by the AMF spec we start from 1
+    this->_material_id = 1 + model->materials.size();  // watchout for implicit cast
     return model->add_material(this->_material_id);
 }