diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 4e28378..86a29d1 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -495,7 +495,7 @@
 
 # Pick a "unique" server port in the range 10000-19999, and a proxy port
 PORT_BASE="0000$$"
-PORT_BASE="$( echo -n $PORT_BASE | tail -c 5 )"
+PORT_BASE="$( printf $PORT_BASE | tail -c 4 )"
 SRV_PORT="1$PORT_BASE"
 PXY_PORT="2$PORT_BASE"
 unset PORT_BASE