diff --git a/NEWS.md b/NEWS.md
index b7b1f11..f430c57 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -3,6 +3,7 @@
 * add new feature: winner take one method for `biproporz()` and `pukelsheim` (#10)
 * allow non-integer vote counts
 * round divisors to as few digits as possible
+* changed error message for undefined/tied results
 * fixed bugs and added more real data tests
 
 # proporz 1.4.0
diff --git a/R/divisor-check.R b/R/divisor-check.R
index 809d74e..7f0b6d2 100644
--- a/R/divisor-check.R
+++ b/R/divisor-check.R
@@ -9,8 +9,11 @@ check_edge_quotient = function(mtrx_quotient, n_seats, return_indices = FALSE) {
     if(is.nan(quotient_first_without)) quotient_first_without <- 0
 
     if(quotient_last_with == quotient_first_without) {
-        indices = which(mtrx_quotient == quotient_last_with, arr.ind = TRUE)[,"col"]
-        parties = paste0(indices, collapse = " & ")
+        indices = unique(which(mtrx_quotient == quotient_last_with, arr.ind = TRUE)[,"col"])
+        parties = collapse_names(indices)
+        if(!is.null(colnames(mtrx_quotient))) {
+            parties <- collapse_names(colnames(mtrx_quotient)[indices])
+        }
         stop("Result is undefined, equal quotient for parties: ", parties, call. = FALSE)
     }
 }
diff --git a/R/divisor.R b/R/divisor.R
index 186c6ab..a3ed898 100644
--- a/R/divisor.R
+++ b/R/divisor.R
@@ -40,6 +40,7 @@ highest_averages_method = function(votes, n_seats, divisors) {
 
     # method
     mtrx_votes = matrix(rep(votes, each=n_seats), ncol = n_parties)
+    colnames(mtrx_votes) <- names(votes)
     mtrx_divisors = matrix(rep(divisors, ncol(mtrx_votes)), ncol = n_parties)
 
     mtrx_quotient = mtrx_votes/mtrx_divisors
diff --git a/R/quota.R b/R/quota.R
index bd4f54e..38a4f12 100644
--- a/R/quota.R
+++ b/R/quota.R
@@ -73,8 +73,11 @@ check_equal_entries = function(remainders, ordered_remainders, n_seats_remaining
     remainder_first_without = remainders[ordered_remainders[n_seats_remaining+1]]
 
     if(remainder_last_with == remainder_first_without) {
-        indices = which(remainders == remainder_last_with, arr.ind = TRUE)
-        parties = paste0(indices, collapse = " & ")
+        indices = unique(which(remainders == remainder_last_with, arr.ind = TRUE))
+        parties = collapse_names(indices)
+        if(!is.null(names(remainders))) {
+            parties <- collapse_names(names(remainders)[indices])
+        }
         stop("Result is undefined, equal remainder for parties: ", parties,
              call. = FALSE)
     }
diff --git a/R/utils.R b/R/utils.R
index 064b0aa..1d486fe 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -104,7 +104,11 @@ assert = function(check) {
 }
 
 collapse_names = function(x) {
-    y = paste(x, collapse = "', '")
-    y <- paste0("'", y, "'")
+    if(is.character(x)) {
+        y = paste(x, collapse = "', '")
+        y <- paste0("'", y, "'")
+    } else {
+        y = paste(x, collapse = ", ")
+    }
     return(y)
 }
diff --git a/tests/testthat/test-biproportional.R b/tests/testthat/test-biproportional.R
index b4889e1..482dce1 100644
--- a/tests/testthat/test-biproportional.R
+++ b/tests/testthat/test-biproportional.R
@@ -142,7 +142,10 @@ test_that("undefined result biproportional", {
                  proporz(rowSums(vm), sum(seats), "round"))
 
     expect_error_fixed(upper_apportionment(vm, seats),
-                       "Result is undefined, equal quotient for parties: 4 & 6")
+                       "Result is undefined, equal quotient for parties: 4, 6")
+
+    expect_error_fixed(biproporz(uri2020$votes_matrix, uri2020$seats_vector, quorum_any(any_district = 0.7)),
+                       "Result is undefined, equal quotient for parties: 'CVP', 'SPGB', 'FDP', 'SVP'")
 
     # manual fix (actual implementation depends on rules)
     vm4 <- vm6 <- vm
diff --git a/tests/testthat/test-proporz.R b/tests/testthat/test-proporz.R
index d229edd..88e5b9c 100644
--- a/tests/testthat/test-proporz.R
+++ b/tests/testthat/test-proporz.R
@@ -86,11 +86,16 @@ test_that("all method names", {
 
 test_that("undefined result errors", {
     expect_error(proporz(c(1, 10, 10), 1, "round"),
-                 "Result is undefined, equal quotient for parties: 2 & 3", fixed = TRUE)
+                 "Result is undefined, equal quotient for parties: 2, 3", fixed = TRUE)
+    expect_error(proporz(c(1, X = 10, ABC = 10), 1, "round"),
+                 "Result is undefined, equal quotient for parties: 'X', 'ABC'", fixed = TRUE)
     expect_equal(proporz(c(1, 10, 10), 2, "round"), c(0,1,1))
 
     expect_error(largest_remainder_method(c(10, 10, 0), 1),
-                 "Result is undefined, equal remainder for parties: 1 & 2",
+                 "Result is undefined, equal remainder for parties: 1, 2",
+                 fixed = TRUE)
+    expect_error(largest_remainder_method(c(A = 10, X = 10, P = 0), 1),
+                 "Result is undefined, equal remainder for parties: 'A', 'X'",
                  fixed = TRUE)
 })
 
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index b77f4db..0685f33 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -53,3 +53,10 @@ test_that("print", {
     M = biproporz(matrix(c(51,60,63,98,100,102,45,120,144), nrow = 3), 4:6)
     expect_equal(capture.output(print(M)), capture.output(print(as.matrix(M))))
 })
+
+test_that("collapse_names", {
+    x = c("Abc", "XYZ")
+    y = c(2, 6, 67)
+    expect_equal(collapse_names(x), "'Abc', 'XYZ'")
+    expect_equal(collapse_names(y), "2, 6, 67")
+})