diff --git a/macro/machine/M4000.g b/macro/machine/M4000.g
index 71a8255..3abbd9e 100644
--- a/macro/machine/M4000.g
+++ b/macro/machine/M4000.g
@@ -15,6 +15,9 @@ if { !inputs[state.thisInput].active }
 if { !exists(param.P) || !exists(param.R) || !exists(param.S) }
     abort { "Must provide tool number (P...), radius (R...) and description (S...) to register tool!" }
 
+if { #param.S < 1 }
+    abort { "Tool description must be at least 1 character long!" }
+
 ; Validate tool index
 if { param.P >= limits.tools || param.P < 0 }
     abort { "Tool index must be between 0 and " ^ limits.tools-1 ^ "!" }
@@ -23,18 +26,25 @@ if { param.P >= limits.tools || param.P < 0 }
 ; This allows us to re-run a file that defines the tool that is currently
 ; loaded, without unloading the tool.
 ; This has to be split over multiple lines due to length of the condition.
-if { exists(global.mosTT[param.P]) && global.mosTT[param.P] != null && exists(tools[param.P]) && tools[param.P] != null }
-    var toolSame = { global.mosTT[param.P][0] == param.R && tools[param.P].spindle == ((exists(param.I)) ? param.I : global.mosSID) }
+; Make sure to check that the tool table has enough entries.
+
+; Initial tool similarity check - make sure the tool is defined in both the internal
+; RRF tool table and our own mosTT table.
+var toolSame = { global.mosTT[param.P] != null && #tools > param.P && tools[param.P] != null }
 
-    set var.toolSame = { var.toolSame && tools[param.P].name == param.S }
+; Check that tool radius and spindle match
+set var.toolSame = { var.toolSame && global.mosTT[param.P][0] == param.R && tools[param.P].spindle == ((exists(param.I)) ? param.I : global.mosSID) }
 
-    if { exists(param.X) }
-        set var.toolSame = { var.toolSame && global.mosTT[param.P][1][0] == param.X }
-    if { exists(param.Y) }
-        set var.toolSame = { var.toolSame && global.mosTT[param.P][1][1] == param.Y }
+; Check that tool description matches
+set var.toolSame = { var.toolSame && tools[param.P].name == param.S }
 
-    if { var.toolSame }
-        M99
+; Check that deflection values match
+set var.toolSame = { var.toolSame && exists(param.X) && global.mosTT[param.P][1][0] == param.X }
+set var.toolSame = { var.toolSame && exists(param.Y) && global.mosTT[param.P][1][1] == param.Y }
+
+; If the tool already matches, return.
+if { var.toolSame }
+    M99
 
 ; Define RRF tool against spindle.
 ; Allow spindle ID to be overridden where necessary using I parameter.
diff --git a/post-processors/freecad/millennium_os_post.py b/post-processors/freecad/millennium_os_post.py
index a6b93be..ef1a7f3 100644
--- a/post-processors/freecad/millennium_os_post.py
+++ b/post-processors/freecad/millennium_os_post.py
@@ -639,6 +639,9 @@ def addtool(self, index, name, params):
         if index in self.tools and name != self.tools[index]['name']:
             raise ValueError("Duplicate tool index {} with different descriptions!".format(index))
 
+        if len(name) < 1:
+            raise ValueError("Tool name must not be empty!")
+
         self.tools[index] = {"name": name, "params": params}
 
     # Return tool info
diff --git a/post-processors/fusion-360/millennium-os.cps b/post-processors/fusion-360/millennium-os.cps
index a8951f0..f49c1b2 100644
--- a/post-processors/fusion-360/millennium-os.cps
+++ b/post-processors/fusion-360/millennium-os.cps
@@ -465,6 +465,9 @@ function onOpen() {
     writeComment("Pass tool details to firmware");
     for(var i = 0; i < nTools; i++) {
       var tool = tools.getTool(i);
+      if(tool.description.length < 1) {
+        error("Tool description must not be empty!");
+      }
       writeBlock('{cmd} P{index} R{radius} S"{desc}"'.supplant({
         cmd: mCodes.format(M.ADD_TOOL),
         index: intFmt.format(tool.number),