diff --git a/src/52-fsession-latency/fsession_latency.c b/src/52-fsession-latency/fsession_latency.c
index a4219f2..e5c6e7c 100644
--- a/src/52-fsession-latency/fsession_latency.c
+++ b/src/52-fsession-latency/fsession_latency.c
@@ -63,7 +63,7 @@ static int parse_u64(const char *value, unsigned long long maximum,
 
 	errno = 0;
 	parsed = strtoull(value, &end, 10);
-	if (errno || !end || *end || parsed > maximum)
+	if (errno || end == value || *end || parsed > maximum)
 		return -EINVAL;
 	*result = parsed;
 	return 0;
diff --git a/src/52-fsession-latency/tests/test_fsession_latency.py b/src/52-fsession-latency/tests/test_fsession_latency.py
index f04933c..22e41e8 100644
--- a/src/52-fsession-latency/tests/test_fsession_latency.py
+++ b/src/52-fsession-latency/tests/test_fsession_latency.py
@@ -101,6 +101,16 @@ def main() -> int:
     assert invalid.returncode != 0
     assert "invalid duration" in invalid.stdout
 
+    empty_threshold = subprocess.run(
+        [binary, "--threshold-us", ""],
+        text=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        check=False,
+    )
+    assert empty_threshold.returncode != 0
+    assert "invalid threshold" in empty_threshold.stdout
+
     pid = str(os.getpid())
     missed, _ = run_trace(
         binary,