diff --git a/src/frontend/mosh-client.cc b/src/frontend/mosh-client.cc
index fa5114b..a9ab798 100644
--- a/src/frontend/mosh-client.cc
+++ b/src/frontend/mosh-client.cc
@@ -73,7 +73,7 @@ int main( int argc, char *argv[] )
     }
   }
 
-  char *ip;
+  char *ip, *desired_port;
   int port;
 
   if ( argc - optind != 2 ) {
@@ -82,7 +82,24 @@ int main( int argc, char *argv[] )
   }
 
   ip = argv[ optind ];
-  port = myatoi( argv[ optind + 1 ] );
+  desired_port = argv[ optind + 1 ];
+
+  /* Sanity-check arguments */
+  if ( ip
+       && ( strspn( ip, "0123456789." ) != strlen( ip ) ) ) {
+    fprintf( stderr, "%s: Bad IP address (%s)\n\n", argv[ 0 ], ip );
+    usage( argv[ 0 ] );
+    exit( 1 );
+  }
+
+  if ( desired_port
+       && ( strspn( desired_port, "0123456789" ) != strlen( desired_port ) ) ) {
+    fprintf( stderr, "%s: Bad UDP port (%s)\n\n", argv[ 0 ], desired_port );
+    usage( argv[ 0 ] );
+    exit( 1 );
+  }
+
+  port = myatoi( desired_port );
 
   /* Read key from environment */
   char *env_key = getenv( "MOSH_KEY" );