diff --git a/src/Mod/CAM/Path/Op/Gui/Base.py b/src/Mod/CAM/Path/Op/Gui/Base.py
index 2cda1f989e..b492e0a1d3 100644
--- a/src/Mod/CAM/Path/Op/Gui/Base.py
+++ b/src/Mod/CAM/Path/Op/Gui/Base.py
@@ -1366,7 +1366,10 @@ class TaskPanel(object):
 
         # Update properties based upon expressions in case expression value has changed
         for prp, expr in self.obj.ExpressionEngine:
-            val = FreeCAD.Units.Quantity(self.obj.evalExpression(expr))
+            evalExpr = self.obj.evalExpression(expr)
+            if not isinstance(evalExpr, (int, float, FreeCAD.Units.Quantity)):
+                continue
+            val = FreeCAD.Units.Quantity(evalExpr)
             value = val.Value if hasattr(val, "Value") else val
             prop = getattr(self.obj, prp)
             if hasattr(prop, "Value"):