diff --git a/gui/wxpython/core/menutree.py b/gui/wxpython/core/menutree.py
index ef3ebd7b414..a418ffcd98a 100644
--- a/gui/wxpython/core/menutree.py
+++ b/gui/wxpython/core/menutree.py
@@ -256,7 +256,7 @@ def collectParents(node, parents):
     else:
         import grass.script.core as gscore
 
-        gscore.fatal("Unknown value for parameter menu: " % menu)
+        gscore.fatal("Unknown value for parameter menu: %s" % menu)
 
     if action == "strings":
         menudata.PrintStrings(sys.stdout)
@@ -269,6 +269,6 @@ def collectParents(node, parents):
     else:
         import grass.script.core as gscore
 
-        gscore.fatal("Unknown value for parameter action: " % action)
+        gscore.fatal("Unknown value for parameter action: %s" % action)
 
     sys.exit(0)