diff --git a/qpms/qpms_c.pyx b/qpms/qpms_c.pyx
index d05b5ed..3adb2c9 100644
--- a/qpms/qpms_c.pyx
+++ b/qpms/qpms_c.pyx
@@ -611,7 +611,7 @@ cdef class ScatteringSystem:
         cdef intptr_t offset = 0
         for pi in range(self.s[0].p_count):
             ar_view[pi] = offset
-            offset += self.s[0].tm[self.s[0].p[pi].tmatrix_id].spec[0].n
+            offset += qpms_ss_bspec_pi(self.s, pi)[0].n
         return ar
 
     property positions:
diff --git a/qpms/qpms_cdefs.pxd b/qpms/qpms_cdefs.pxd
index bb4cf9b..d5c515d 100644
--- a/qpms/qpms_cdefs.pxd
+++ b/qpms/qpms_cdefs.pxd
@@ -624,6 +624,7 @@ cdef extern from "scatsystem.h":
     beyn_result_t *qpms_scatsys_periodic_find_eigenmodes(const qpms_scatsys_t *ss, const double *k,
             cdouble omega_centre, double omega_rr, double omega_ri, size_t contour_npoints, 
             double rank_tol, size_t rank_sel_min, double res_tol)
+    const qpms_vswf_set_spec_t *qpms_ss_bspec_pi(const qpms_scatsys_t *ss, qpms_ss_pi_t pi) 
 
 cdef extern from "ewald.h":
     struct qpms_csf_result: