diff --git a/source/helper.c b/source/helper.c
index 2d6956f1..0795bdfe 100644
--- a/source/helper.c
+++ b/source/helper.c
@@ -573,6 +573,12 @@ void config_sanity_check ( Display *display )
             found_error        = 1;
         }
     }
+    if ( config.fullscreen && config.monitor != -1 ) {
+        g_string_append_printf ( msg, "\t<b>config.monitor</b>=%d is invalid. Value should be unset (-1) when fullscreen mode is enabled.\n",
+            config.monitor);
+        config.monitor = -1;
+        found_error        = 1;
+    }
 
     // Check size
     {