diff --git a/qpms/lattices2d.c b/qpms/lattices2d.c
index 4b94929..ee99d06 100644
--- a/qpms/lattices2d.c
+++ b/qpms/lattices2d.c
@@ -941,7 +941,7 @@ void qpms_emptylattice2_modes_nearest(double target[2],
   double *freqlist;
   size_t n = qpms_emptylattice2_modes_maxfreq(&freqlist,
       b1, b2, rtol, k, c, omega);
-  target[0] = freqlist[n-2];
+  target[0] = (n > 1) ? freqlist[n-2] : NAN;
   target[1] = freqlist[n-1];
   free(freqlist);
 }
diff --git a/qpms/qpms_cdefs.pxd b/qpms/qpms_cdefs.pxd
index 940c7f0..48b1a39 100644
--- a/qpms/qpms_cdefs.pxd
+++ b/qpms/qpms_cdefs.pxd
@@ -211,6 +211,11 @@ cdef extern from "lattices.h":
             double maxR, bint inc_maxR, PGen_1D_incrementDirection incdir)
     int qpms_reduce_lattice_basis(double *b, size_t bsize, size_t ndim, double delta)
 
+    size_t qpms_emptylattice2_modes_maxfreq(double **target_freqs, cart2_t b1_rec, cart2_t b2_rec,
+            double rtol, cart2_t k, double wave_speed, double maxomega)
+    size_t qpms_emptylattice2_modes_nearest(double *target_freqs, cart2_t b1_rec, cart2_t b2_rec,
+            double rtol, cart2_t k, double wave_speed, double omega)
+
 
 cdef extern from "quaternions.h":
     qpms_quat_t qpms_quat_2c_from_4d(qpms_quat4d_t q)