diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 3103be8a32..21225330c2 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -905,7 +905,13 @@ test_expect_code () {
 # - not all diff versions understand "-u"
 
 test_cmp() {
-	eval "$GIT_TEST_CMP" '"$@"'
+	test $# -eq 2 || BUG "test_cmp requires two arguments"
+	if ! eval "$GIT_TEST_CMP" '"$@"'
+	then
+		test "x$1" = x- || test -e "$1" || BUG "test_cmp '$1' missing"
+		test "x$2" = x- || test -e "$2" || BUG "test_cmp '$2' missing"
+		return 1
+	fi
 }
 
 # Check that the given config key has the expected value.
@@ -934,7 +940,13 @@ test_cmp_config() {
 # test_cmp_bin - helper to compare binary files
 
 test_cmp_bin() {
-	cmp "$@"
+	test $# -eq 2 || BUG "test_cmp_bin requires two arguments"
+	if ! cmp "$@"
+	then
+		test "x$1" = x- || test -e "$1" || BUG "test_cmp_bin '$1' missing"
+		test "x$2" = x- || test -e "$2" || BUG "test_cmp_bin '$2' missing"
+		return 1
+	fi
 }
 
 # Use this instead of test_cmp to compare files that contain expected and