diff --git a/macro/machine/M3.9.g b/macro/machine/M3.9.g
index 14a2af8..48736f4 100644
--- a/macro/machine/M3.9.g
+++ b/macro/machine/M3.9.g
@@ -19,6 +19,10 @@ if { exists(param.P) && param.P < 0 }
 ; Allocate Spindle ID
 var sID = { (exists(param.P) ? param.P : global.mosSID) }
 
+; Validate Spindle ID
+if { var.sID < 0 || var.sID > #spindles-1 || spindles[var.sID] == null || spindles[var.sID].state == "unconfigured" }
+    abort { "Spindle ID " ^ var.sID ^ " is not valid!" }
+
 ; Validate Spindle Speed parameter
 if { exists(param.S) }
     if { param.S < 0 }
diff --git a/macro/machine/M5.9.g b/macro/machine/M5.9.g
index 4481bd2..23772a3 100644
--- a/macro/machine/M5.9.g
+++ b/macro/machine/M5.9.g
@@ -30,6 +30,10 @@ var sID = { global.mosSID }
 var doWait = false
 
 while { (iterations < #spindles) && !var.doWait }
+    ; Ignore unconfigured spindles
+    if { spindles[iterations].state == "unconfigured" }
+        continue
+
     set var.doWait = { spindles[iterations].current != 0 }
     ; In case M5.9 should stop a spindle that _isnt_ the one
     ; configured in MOS. We'll calculate the delay time based
@@ -53,7 +57,13 @@ elif { var.doWait }
     ; by that percentage with 5% extra leeway.
     ; Ceil this so we always wait a round second, no point waiting
     ; less than 1 anyway.
-    set var.dwellTime = { ceil(var.dwellTime * (abs(spindles[var.sID].current) / spindles[var.sID].max) * 1.05) }
+
+    ; We want to run M5 regardless of if var.sID is valid or not, so we check
+    ; for nulls on the individual values before doing the dwellTime calculation.
+    ; If the current spindle is not valid then M5 will be called but we wont
+    ; wait for it to stop.
+    if { spindles[var.sID].current != null && spindles[var.sID].max != null }
+        set var.dwellTime = { ceil(var.dwellTime * (abs(spindles[var.sID].current) / spindles[var.sID].max) * 1.05) }
 
 ; We run M5 unconditionally for safety purposes. If
 ; the object model is not up to date for whatever