diff --git a/run-vmtest/normalize_bpf_test_names.py b/run-vmtest/normalize_bpf_test_names.py
index b8a88286..d32aec51 100755
--- a/run-vmtest/normalize_bpf_test_names.py
+++ b/run-vmtest/normalize_bpf_test_names.py
@@ -45,7 +45,10 @@ def next_test(lines: list[str]):
     prev = lines[0]
 
     def is_subtest(line: str) -> bool:
-        return ('/' in line) and ('/' not in prev) and line.startswith(prev)
+        # Compare against "prev/" rather than "prev": without the separator
+        # "a_test" also swallows "a_test_two/subtest", which is a subtest of a
+        # different test that happens to share a prefix.
+        return ('/' not in prev) and line.startswith(prev + '/')
 
     yield lines[0]
     for line in lines[1:]:
diff --git a/run-vmtest/tests/normalize_bpf_test_names/expected-output.txt b/run-vmtest/tests/normalize_bpf_test_names/expected-output.txt
index e0029e2b..1533edd4 100644
--- a/run-vmtest/tests/normalize_bpf_test_names/expected-output.txt
+++ b/run-vmtest/tests/normalize_bpf_test_names/expected-output.txt
@@ -10,3 +10,5 @@ test4_blah
 test5
 test6
 test7/subtest123
+test8
+test8_peek/subtest1
diff --git a/run-vmtest/tests/normalize_bpf_test_names/input.txt b/run-vmtest/tests/normalize_bpf_test_names/input.txt
index 5b9a48d0..14fb97e7 100644
--- a/run-vmtest/tests/normalize_bpf_test_names/input.txt
+++ b/run-vmtest/tests/normalize_bpf_test_names/input.txt
@@ -18,3 +18,5 @@ test5                                                   # should consume test5/s
    ## some blank lines, should be removed
       
 
+test8
+test8_peek/subtest1                                     # must be kept: test8_peek is a different test, not a subtest of test8