diff --git a/lib/www/server/lib/headers/hydronav.js b/lib/www/server/lib/headers/hydronav.js
index 2e42157..b6e6a29 100644
--- a/lib/www/server/lib/headers/hydronav.js
+++ b/lib/www/server/lib/headers/hydronav.js
@@ -106,7 +106,7 @@ function parse (buffer) {
 					ctx.speed *= (1.852/3.6);
 					break;
 				default:
-					throw new NavHeaderError("Unexpected speed units: "+units, buf);
+					throw new NavHeaderError("Unexpected speed units: "+units+" for speed: "+ctx.speed, buf);
 			}
 		},