diff --git a/tests/test-webserver.sh b/tests/test-webserver.sh
index 3291b061..2964ce96 100755
--- a/tests/test-webserver.sh
+++ b/tests/test-webserver.sh
@@ -10,9 +10,15 @@ env PYTHONUNBUFFERED=1 setsid python -m SimpleHTTPServer 0 >${test_tmpdir}/httpd
 child_pid=$!
 
 for x in $(seq 50); do
-    sed -e 's,Serving HTTP on 0.0.0.0 port \([0-9]*\) \.\.\.,\1,' < ${test_tmpdir}/httpd-output > ${test_tmpdir}/httpd-port
-    if ! cmp ${test_tmpdir}/httpd-output ${test_tmpdir}/httpd-port 1>/dev/null; then
-        break
+    # Snapshot the output
+    cp ${test_tmpdir}/httpd-output{,.tmp}
+    # If it's non-empty, see whether it matches our regexp
+    if test -s ${test_tmpdir}/httpd-output.tmp; then
+        sed -e 's,Serving HTTP on 0.0.0.0 port \([0-9]*\) \.\.\.,\1,' < ${test_tmpdir}/httpd-output.tmp > ${test_tmpdir}/httpd-port
+        if ! cmp ${test_tmpdir}/httpd-output.tmp ${test_tmpdir}/httpd-port 1>/dev/null; then
+            # If so, we've successfully extracted the port
+            break
+        fi
     fi
     sleep 0.1
 done