diff --git a/Grbl_Esp32/src/GCode.cpp b/Grbl_Esp32/src/GCode.cpp
index 6f8daf15..70cce7d3 100644
--- a/Grbl_Esp32/src/GCode.cpp
+++ b/Grbl_Esp32/src/GCode.cpp
@@ -476,6 +476,7 @@ Error gc_execute_line(char* line, uint8_t client) {
                                 if (spindle->is_reversable || spindle->inLaserMode()) {
                                     gc_block.modal.spindle = SpindleState::Ccw;
                                 } else {
+                                    grbl_msg_sendf(CLIENT_SERIAL, MsgLevel::Info, "M4 requires laser mode or a reversable spindle");
                                     FAIL(Error::GcodeUnsupportedCommand);
                                 }
                                 break;
diff --git a/Grbl_Esp32/src/Stepper.cpp b/Grbl_Esp32/src/Stepper.cpp
index 83aee938..d4919614 100644
--- a/Grbl_Esp32/src/Stepper.cpp
+++ b/Grbl_Esp32/src/Stepper.cpp
@@ -551,6 +551,8 @@ void st_prep_buffer() {
                     prep.current_speed = sqrt(pl_block->entry_speed_sqr);
                 }
 
+                st_prep_block->is_pwm_rate_adjusted = false;  // set default value
+                // prep.inv_rate is only used if is_pwm_rate_adjusted is true
                 if (spindle->inLaserMode()) {  //
                     if (pl_block->spindle == SpindleState::Ccw) {
                         // Pre-compute inverse programmed rate to speed up PWM updating per step segment.