diff --git a/src/assert_failure.bash b/src/assert_failure.bash
index 31f89ce..268d447 100644
--- a/src/assert_failure.bash
+++ b/src/assert_failure.bash
@@ -63,7 +63,7 @@ assert_failure() {
   if (( status == 0 )); then
     { local -ir width=6
       batslib_print_kv_single_or_multi "$width" 'output' "$output"
-      if [[ -n "$stderr" ]]; then
+      if [[ -n "${stderr-}" ]]; then
         batslib_print_kv_single_or_multi "$width" 'stderr' "$stderr"
       fi
     } \
@@ -76,7 +76,7 @@ assert_failure() {
       'actual'   "$status"
       batslib_print_kv_single_or_multi "$width" \
       'output' "$output"
-      if [[ -n "$stderr" ]]; then
+      if [[ -n "${stderr-}" ]]; then
       	batslib_print_kv_single_or_multi "$width" 'stderr' "$stderr"
       fi
     } \
diff --git a/src/assert_success.bash b/src/assert_success.bash
index eaadf95..e3f074d 100644
--- a/src/assert_success.bash
+++ b/src/assert_success.bash
@@ -37,7 +37,7 @@ assert_success() {
     { local -ir width=6
       batslib_print_kv_single "$width" 'status' "$status"
       batslib_print_kv_single_or_multi "$width" 'output' "$output"
-      if [[ -n "$stderr" ]]; then
+      if [[ -n "${stderr-}" ]]; then
       	batslib_print_kv_single_or_multi "$width" 'stderr' "$stderr"
       fi
     } \
diff --git a/test/assert_failure.bats b/test/assert_failure.bats
index 0882c4a..beb108f 100755
--- a/test/assert_failure.bats
+++ b/test/assert_failure.bats
@@ -22,14 +22,14 @@ ERR_MSG
 }
 
 @test "assert_failure(): returns 1 and displays \`\$stderr' if it is set" {
+  bats_require_minimum_version 1.5.0
 	run --separate-stderr \
 			bash -c 'echo "a"
 							 echo "b" >&2
 							 exit 0'
+  echo "Stderr: $stderr" >&3
 	run assert_failure
 
-	echo "Stderr: $stderr" >&3
-
 	assert_test_fail <<'ERR_MSG'
 
 -- command succeeded, but it was expected to fail --
diff --git a/test/assert_success.bats b/test/assert_success.bats
index 5907b66..719c0f7 100755
--- a/test/assert_success.bats
+++ b/test/assert_success.bats
@@ -40,6 +40,7 @@ ERR_MSG
 }
 
 @test "assert_success(): displays \`\$stderr' if it is set" {
+  bats_require_minimum_version 1.5.0
 	run --separate-stderr \
 			bash -c 'echo "a"
 							 echo "b" >&2
diff --git a/test/refute_stderr.bats b/test/refute_stderr.bats
index d1ec134..6f8dfc5 100755
--- a/test/refute_stderr.bats
+++ b/test/refute_stderr.bats
@@ -236,7 +236,7 @@ ERR_MSG
 }
 
 @test "refute_stderr(): \`--' stops parsing options" {
-  run echo_err '--'
+  run --separate-stderr echo_err '--'
   run refute_stderr -- '-p'
   assert_test_pass
 }