diff --git a/core/src/mindustry/world/blocks/units/UnitAssembler.java b/core/src/mindustry/world/blocks/units/UnitAssembler.java
index 569e276984..52d1de976b 100644
--- a/core/src/mindustry/world/blocks/units/UnitAssembler.java
+++ b/core/src/mindustry/world/blocks/units/UnitAssembler.java
@@ -275,7 +275,10 @@ public class UnitAssembler extends PayloadBlock{
                     break;
                 }
             }
-            currentTier = max;
+            if(currentTier != max){
+                progress=0;
+                currentTier = max;
+            }
         }
 
         public UnitType unit(){