diff --git a/src/bindings.cc b/src/bindings.cc
index e92b40e5..fff4b47b 100644
--- a/src/bindings.cc
+++ b/src/bindings.cc
@@ -79,6 +79,8 @@ bool OBBindings::translate(const std::string &str, Binding &b)
     } else if (mod == "M5" ||   // mod5
                mod == "Mod5") {
       mods |= Mod5Mask;
+    } else {                    // invalid
+      return false;
     }
     begin = end + 1;
   }