diff --git a/util-networks.R b/util-networks.R
index 1edbb4ab..c6d81305 100644
--- a/util-networks.R
+++ b/util-networks.R
@@ -1282,6 +1282,14 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
             ## combine the networks:
             ## 1) merge the existing networks
             u = igraph::disjoint_union(authors.net, artifacts.net)
+            for (attr in igraph::edge_attr_names(u)) {
+                values = igraph::edge_attr(u, attr)
+                NULLs = sapply(values, is.null)
+                if (any(NULLs)) {
+                    values[NULLs] = NA
+                    u = igraph::set_edge_attr(u, attr, value = values)
+                }
+            }
 
             ## As there is a bug in 'igraph::disjoint_union' in igraph from its version 1.4.0 on, which is still
             ## present, at least, until its version 2.0.3 (see https://github.com/igraph/rigraph/issues/761), we need
@@ -1806,6 +1814,15 @@ add.edges.for.bipartite.relation = function(net, bipartite.relations, network.co
 
         ## add the vertex sequences as edges to the network
         net = igraph::add_edges(net, unlist(vertex.sequence.for.edges), attr = extra.edge.attributes)
+
+        for (attr in igraph::edge_attr_names(net)) {
+            values = igraph::edge_attr(net, attr)
+            NULLs = sapply(values, is.null)
+            if (any(NULLs)) {
+                values[NULLs] = NA
+                net = igraph::set_edge_attr(net, attr, value = values)
+            }
+        }
     }
 
     return(net)