diff --git a/src/uan/model/uan-phy-gen.cc b/src/uan/model/uan-phy-gen.cc
index d84d7a9388..4125f9f86f 100644
--- a/src/uan/model/uan-phy-gen.cc
+++ b/src/uan/model/uan-phy-gen.cc
@@ -334,7 +334,7 @@ UanPhyPerCommonModes::CalcPer(Ptr<Packet> pkt, double sinrDb, UanTxMode mode)
             // Eq (74)
             for (int j = 0; j < sum_items; ++j)
             {
-                PbK += ::std::pow(-1.0, (double)j * pow2k / sqrtM) *
+                PbK += ::std::pow(-1.0, ::std::floor((double)j * pow2k / sqrtM)) *
                        (pow2k - ::std::floor((double)(j * pow2k / sqrtM) + 0.5)) *
                        erfc((2.0 * (double)j + 1.0) *
                             ::std::sqrt(3.0 * (log2M * EbNo) / (2.0 * (M - 1.0))));