Index: lib/fuzzer/scripts/collect_data_flow.py
===================================================================
--- lib/fuzzer/scripts/collect_data_flow.py
+++ lib/fuzzer/scripts/collect_data_flow.py
@@ -63,13 +63,26 @@
     r = q.pop()
     print("******* Trying:  ", r)
     tmpfile = os.path.join(tmpdir, str(r[0]) + "-" + str(r[1]))
-    ret = subprocess.call([exe, str(r[0]), str(r[1]), inp, tmpfile])
+
+    proc = subprocess.Popen([exe, str(r[0]), str(r[1]), inp, tmpfile],
+                            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    output, err = proc.communicate()
+    if b'FATAL: DataFlowSanitizer: out of labels' in err:
+      print('The input is too long, skipping.')
+      break
+
+    ret = proc.returncode
     if ret and r[1] - r[0] >= 2:
       q.append([r[0], (r[1] + r[0]) / 2])
       q.append([(r[1] + r[0]) / 2, r[1]])
     else:
       outputs.append(tmpfile)
       print("******* Success: ", r)
+
+  if len(outputs) == 0:
+    print('No traces to merge. Exiting.')
+    sys.exit(1)
+
   f = sys.stdout
   if len(argv) >= 4:
     f = open(argv[3], "w")
Index: test/fuzzer/dataflow.test
===================================================================
--- test/fuzzer/dataflow.test
+++ test/fuzzer/dataflow.test
@@ -70,8 +70,8 @@
 RUN: %t-ExplodeDFSanLabelsTestDF 0 2  %t/IN/1234567890123456
 RUN: %t-ExplodeDFSanLabelsTestDF 2 4  %t/IN/1234567890123456
 RUN: %t-ExplodeDFSanLabelsTestDF 4 6  %t/IN/1234567890123456
-# Or we can use collect_data_flow
-RUN: %libfuzzer_src/scripts/collect_data_flow.py %t-ExplodeDFSanLabelsTestDF %t/IN/1234567890123456
+# Or we can use collect_data_flow and fail as well
+RUN: not %libfuzzer_src/scripts/collect_data_flow.py %t-ExplodeDFSanLabelsTestDF %t/IN/1234567890123456
 
 # Test that we can run collect_data_flow on the entire corpus dir
 RUN: rm -rf %t/OUT