diff --git a/WFObj.cc b/WFObj.cc
index 0a31162..c358795 100644
--- a/WFObj.cc
+++ b/WFObj.cc
@@ -462,6 +462,12 @@ bool WFObj::buildVBO()
             it != m_faces.end();
             it++)
     {
+        if (m_materials.find(it->first) == m_materials.end())
+        {
+            cerr << "Material '" << it->first << "' referenced in '"
+                << m_path << "' is not defined." << endl;
+            return false;
+        }
         for (vector<Face>::iterator fit = it->second.begin();
                 fit != it->second.end();
                 fit++)