diff --git a/klippy/toolhead.py b/klippy/toolhead.py
index 64790130..ef9368fc 100644
--- a/klippy/toolhead.py
+++ b/klippy/toolhead.py
@@ -467,13 +467,17 @@ class ToolHead:
         gcode.respond_info(msg, log=False)
     def cmd_M204(self, params):
         gcode = self.printer.lookup_object('gcode')
-        if 'P' in params and 'T' in params and 'S' not in params:
+        if 'S' in params:
+            # Use S for accel
+            accel = gcode.get_float('S', params, above=0.)
+        elif 'P' in params and 'T' in params:
             # Use minimum of P and T for accel
             accel = min(gcode.get_float('P', params, above=0.),
                         gcode.get_float('T', params, above=0.))
         else:
-            # Use S for accel
-            accel = gcode.get_float('S', params, above=0.)
+            gcode.respond_info('Invalid M204 command "%s"'
+                               % (params['#original'],))
+            return
         self.max_accel = min(accel, self.config_max_accel)
         self._calc_junction_deviation()