diff --git a/tests/functional/common/vars-and-functions.sh b/tests/functional/common/vars-and-functions.sh
index 4316a30d5..e5cc04bb3 100644
--- a/tests/functional/common/vars-and-functions.sh
+++ b/tests/functional/common/vars-and-functions.sh
@@ -236,7 +236,8 @@ expect() {
     expected="$1"
     shift
     "$@" && res=0 || res="$?"
-    if [[ $res -ne $expected ]]; then
+    # also match "negative" codes, which wrap around to >127
+    if [[ $res -ne $expected && $res -ne $[256 + expected] ]]; then
         echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
@@ -250,7 +251,8 @@ expectStderr() {
     expected="$1"
     shift
     "$@" 2>&1 && res=0 || res="$?"
-    if [[ $res -ne $expected ]]; then
+    # also match "negative" codes, which wrap around to >127
+    if [[ $res -ne $expected && $res -ne $[256 + expected] ]]; then
         echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
diff --git a/tests/functional/test-infra.sh b/tests/functional/test-infra.sh
index 37322b356..f6f84eae9 100755
--- a/tests/functional/test-infra.sh
+++ b/tests/functional/test-infra.sh
@@ -13,6 +13,25 @@ expect 1 false
 # `expect` will fail when we get it wrong
 expect 1 expect 0 false
 
+function ret() {
+  return $1
+}
+
+# `expect` can call functions, not just executables
+expect 0 ret 0
+expect 1 ret 1
+
+# `expect` supports negative exit codes
+expect -1 ret -1
+
+# or high positive ones, equivalent to negative ones
+expect 255 ret 255
+expect 255 ret -1
+expect -1 ret 255
+
+# but it doesn't confuse negative exit codes with positive ones
+expect 1 expect -10 ret 10
+
 noisyTrue () {
     echo YAY! >&2
     true