Merge pull request #872 from flying-sheep/fix-get-adjacency-sparse

Author: ntamas

src/igraph/adjacency.py

@@ -113,7 +113,8 @@ def _get_adjacency_sparse(self, attribute=None):
         weights = self.es[attribute]
 
     N = self.vcount()
-    mtx = sparse.csr_matrix((weights, list(zip(*edges))), shape=(N, N))
+    r, c = zip(*edges) if edges else ([], [])
+    mtx = sparse.csr_matrix((weights, (r, c)), shape=(N, N))
 
     if not self.is_directed():
         mtx = mtx + sparse.triu(mtx, 1).T + sparse.tril(mtx, -1).T