diff --git a/src/brep/brep.cpp b/src/brep/brep.cpp
index 2fe969d..cc284c6 100644
--- a/src/brep/brep.cpp
+++ b/src/brep/brep.cpp
@@ -59,16 +59,24 @@ core::AABB3D BrepModel::bounds() const {
 }
 
 bool BrepModel::is_valid() const {
-    // Basic validity: all referenced IDs must exist
     for (const auto& e : edges_) {
         if (e.v_start >= static_cast<int>(vertices_.size()) ||
-            e.v_end >= static_cast<int>(vertices_.size()))
+            e.v_end >= static_cast<int>(vertices_.size())) {
+            fprintf(stderr, "is_valid FAIL: edge v=%d/%d vs %zu verts\n", e.v_start, e.v_end, vertices_.size());
             return false;
+        }
     }
     for (const auto& f : faces_) {
-        if (f.surface_id >= static_cast<int>(surfaces_.size())) return false;
-        for (int li : f.loops)
-            if (li >= static_cast<int>(loops_.size())) return false;
+        if (f.surface_id >= static_cast<int>(surfaces_.size())) {
+            fprintf(stderr, "is_valid FAIL: face.surface_id=%d vs %zu surfaces\n", f.surface_id, surfaces_.size());
+            return false;
+        }
+        for (int li : f.loops) {
+            if (li >= static_cast<int>(loops_.size())) {
+                fprintf(stderr, "is_valid FAIL: loop=%d vs %zu loops\n", li, loops_.size());
+                return false;
+            }
+        }
     }
     return true;
 }