diff --git a/sanic/app.py b/sanic/app.py
index 3776c915..5d80b360 100644
--- a/sanic/app.py
+++ b/sanic/app.py
@@ -460,7 +460,10 @@ class Sanic:
 
         if external:
             if not scheme:
-                scheme = netloc[:8].split(':', 1)[0]
+                if ':' in netloc[:8]:
+                    scheme = netloc[:8].split(':', 1)[0]
+                else:
+                    scheme = 'http'
 
             if '://' in netloc[:8]:
                 netloc = netloc.split('://', 1)[-1]