diff --git a/opm/autodiff/WellStateFullyImplicitBlackoil.hpp b/opm/autodiff/WellStateFullyImplicitBlackoil.hpp
index 353ff201b..edfcca5ae 100644
--- a/opm/autodiff/WellStateFullyImplicitBlackoil.hpp
+++ b/opm/autodiff/WellStateFullyImplicitBlackoil.hpp
@@ -456,9 +456,13 @@ namespace Opm
             {
                 // we need to create a trival segment related values to avoid there will be some
                 // multi-segment wells added later.
+                nseg_ = nw;
+                seg_number_.clear();
                 top_segment_index_.reserve(nw);
+                seg_number_.reserve(nw);
                 for (int w = 0; w < nw; ++w) {
                     top_segment_index_.push_back(w);
+                    seg_number_.push_back(1); // Top segment is segment #1
                 }
                 segpress_ = bhp();
                 segrates_ = wellRates();