diff --git a/Phys/SelTools/include/SelTools/DistanceCalculator.h b/Phys/SelTools/include/SelTools/DistanceCalculator.h
index 2d3c139b448cd3db79634c071d14a075100030a7..08b57050d8742e8ccbbe55e19ea5bf616795b36e 100644
--- a/Phys/SelTools/include/SelTools/DistanceCalculator.h
+++ b/Phys/SelTools/include/SelTools/DistanceCalculator.h
@@ -259,9 +259,16 @@ namespace Sel {
       }
 
       auto [success, W_Inv] = W.invChol();
-      auto halfdChi2dLam2   = similarity( dir, W_Inv );
-      auto decayLength      = dot( dir, W_Inv * flight ) / halfdChi2dLam2;
-      auto decayLengthErr   = sqrt( 1 / halfdChi2dLam2 );
+      // Temporary fix: If W is not invertible due to neg Det, exclude 'a' terms.
+      if ( !success ) {
+        for ( size_t row = 0; row < 3; ++row ) {
+          for ( size_t col = 0; col <= row; ++col ) { W( row, col ) = vertexCov( row, col ) + pCov( row, col ); }
+        }
+      }
+      std::tie( success, W_Inv ) = W.invChol();
+      auto halfdChi2dLam2        = similarity( dir, W_Inv );
+      auto decayLength           = dot( dir, W_Inv * flight ) / halfdChi2dLam2;
+      auto decayLengthErr        = sqrt( 1 / halfdChi2dLam2 );
       if constexpr ( std::is_arithmetic_v<decltype( success )> ) {
         return success ? decayLength / decayLengthErr : NonPhysicalValue;
       } else {
@@ -318,11 +325,13 @@ namespace Sel {
       using LHCb::Event::fourMomentum;
       using Sel::Utils::all;
       using std::abs;
+      using std::isnan;
       using std::sqrt;
 
       // convergence parameters
       const float_v delta_chi2 = 0.001;
       const int     m_max_iter = 5;
+      const auto    ctau_init  = ctau;
 
       // invariants which are not changed during iteration
       const auto momCov        = momCovMatrix( particle );
@@ -349,7 +358,9 @@ namespace Sel {
         error     = new_error;
         if ( converged ) { break; }
       }
-
+      // Temporary fix: In case of NaN, take initial value for ctau
+      float ctau_float = static_cast<float>( ctau );
+      ctau             = isnan( ctau_float ) ? ctau_init : ctau;
       if ( !converged && m_no_convergence.has_value() ) { ++( *m_no_convergence ); }
     }