diff --git a/qpms/qpms_error.h b/qpms/qpms_error.h
index b1ea0f7..33597ab 100644
--- a/qpms/qpms_error.h
+++ b/qpms/qpms_error.h
@@ -27,4 +27,6 @@ void qpms_pr_debug_at_flf(const char *filename, unsigned int linenum,
 
 #define QPMS_ENSURE_SUCCESS(x) {if(x) QPMS_WTF;}
 
+#define QPMS_ENSURE(x, msg, ...) {if(!(x)) qpms_pr_error_at_flf(__FILE__,__LINE__,__func__,msg, ##__VA_ARGS__); }
+
 #endif 
diff --git a/qpms/scatsystem.c b/qpms/scatsystem.c
index 27a1ad2..062fba4 100644
--- a/qpms/scatsystem.c
+++ b/qpms/scatsystem.c
@@ -467,16 +467,18 @@ qpms_scatsys_t *qpms_scatsys_apply_symmetry(const qpms_scatsys_t *orig, const qp
         break;
       }
     if (j == ss->tm_count) { // duplicity not found, copy the t-matrix
-      ss->tm[i] = qpms_tmatrix_copy(orig->tm[i]);
+      ss->tm[j] = qpms_tmatrix_copy(orig->tm[i]);
+      ss->max_bspecn = MAX(ss->tm[j]->spec->n, ss->max_bspecn);
+      lMax = MAX(lMax, ss->tm[j]->spec->lMax);
       ++(ss->tm_count);
     } 
     tm_dupl_remap[i] = j;
-    ss->max_bspecn = MAX(ss->tm[i]->spec->n, ss->max_bspecn);
     if (normalisation == QPMS_NORMALISATION_UNDEF)
       normalisation = ss->tm[i]->spec->norm;
     // We expect all bspec norms to be the same.
-    else assert(normalisation == ss->tm[i]->spec->norm); 
-    lMax = MAX(lMax, ss->tm[i]->spec->lMax);
+    else QPMS_ENSURE(normalisation == ss->tm[j]->spec->norm,
+        "Normalisation convention must be the same for all T-matrices."
+        " %d != %d\n", normalisation, ss->tm[j]->spec->norm);
   }
 
   // Copy particles, remapping the t-matrix indices