diff --git a/lib/actions/concordance.py b/lib/actions/concordance.py
index e38674a845..7d558d8aac 100644
--- a/lib/actions/concordance.py
+++ b/lib/actions/concordance.py
@@ -615,7 +615,7 @@ def append_filter(idx: int, attrname, items, ctx, fctxtype) -> int:
         ag_op_idx = 1  # an initial index of auto-generated conc. operations
         ag_op_idx = append_filter(ag_op_idx,
                                   lemmaattr,
-                                  data.fc_lemword.split(),
+                                  data.fc_lemword.split() if data.fc_lemword else None,
                                   (data.fc_lemword_wsize[0], data.fc_lemword_wsize[1], 1),
                                   data.fc_lemword_type)
         append_filter(ag_op_idx,
diff --git a/public/files/js/multidict.ts b/public/files/js/multidict.ts
index e59868263e..f3f137e945 100644
--- a/public/files/js/multidict.ts
+++ b/public/files/js/multidict.ts
@@ -37,7 +37,9 @@ export class MultiDict<T={[k:string]:string|number|boolean}> implements Kontext.
                 if (this.data[k] === undefined) {
                     this.data[k] = [];
                 }
-                this.data[k].push(this.importValue(v));
+                if (this.importValue(v) !== undefined) {
+                    this.data[k].push(this.importValue(v));
+                }
             }
         }
     }