diff -up src/sage/interfaces/ecm.py.orig src/sage/interfaces/ecm.py
--- src/sage/interfaces/ecm.py.orig	2018-12-22 16:37:08.000000000 -0700
+++ src/sage/interfaces/ecm.py	2019-01-15 10:26:31.060040736 -0700
@@ -256,7 +256,7 @@ class ECM(SageObject):
         """
         print("Enter numbers to run ECM on them.")
         print("Press control-C to exit.")
-        os.system(self._cmd)
+        os.system(' '.join(self._cmd))
 
     # Recommended settings from
     # http://www.mersennewiki.org/index.php/Elliptic_Curve_Method