diff --git a/tests/ut_seash_subprocess.py b/tests/ut_seash_subprocess.py
index <HASH>..<HASH> 100644
--- a/tests/ut_seash_subprocess.py
+++ b/tests/ut_seash_subprocess.py
@@ -14,6 +14,17 @@ initproc.wait()
 
 nmproc = subprocess.Popen([sys.executable, "nmmain.py", "--test-mode"], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 
+# Make sure the nodemanager (or its parent Python interpreter) doesn't 
+# print anything on stderr before we continue. Otherwise, the test 
+# nodemanager probably didn't start, and subsequent unit tests will 
+# surely hang/fail. See SeattleTestbed/seash#80.
+
+output, error = nmproc.communicate()
+
+if error is not "":
+  raise Exception("Error starting nodemanager in test mode: '" +
+      error + "'")
+
 # without closing stdin, stdout, stderr, nmmain.py won't execute on XP
 nmproc.stdin.close()
 nmproc.stdout.close()
Raise error if test nodemanager didn't come up

We try to detect this by looking at its `stderr`. See SeattleTestbed/seash#<I> (and SeattleTestbed/utf#<I>) why it's important to fail on such errors.