diff --git a/qpms/ewald.c b/qpms/ewald.c
index eb8f57b..07f3892 100644
--- a/qpms/ewald.c
+++ b/qpms/ewald.c
@@ -317,7 +317,7 @@ int ewald32_sigma_short_shiftedpoints(
         if((m+n) % 2 != 0) // odd coefficients are zero.
           continue; // nothing needed, already done by memset
         complex double e_imf = cexp(I*m*Rpq_shifted_arg);
-        double leg = c->legendre0[gsl_sf_legendre_array_index(n, m)];
+        double leg = c->legendre0[gsl_sf_legendre_array_index(n, abs(m))];
         qpms_y_t y = qpms_mn2y_sc(m,n);
         if(err)
           kahanadd(err + y, err_c + y, cabs(leg * (prefacn / I) * R_pq_pown
diff --git a/tests/ewalds.c b/tests/ewalds.c
index ebcddd1..faf27ec 100644
--- a/tests/ewalds.c
+++ b/tests/ewalds.c
@@ -175,7 +175,7 @@ ewaldtest_triang_results *ewaldtest_triang(const ewaldtest_triang_params p) {
         complex double eimf = cexp(I*m*arg_pq);
         results->regsigmas_416[y] +=
           4*M_PI*ipow(n)/p.k/A 
-          * eimf * legendres[gsl_sf_legendre_array_index(n,m)]
+          * eimf * legendres[gsl_sf_legendre_array_index(n,abs(m))]
           / denom;
       }
     }