diff --git a/src/Neo.Compiler.CSharp/Optimizer/Strategies/Optimizer.cs b/src/Neo.Compiler.CSharp/Optimizer/Strategies/Optimizer.cs
index cfc856a07..de772c8bd 100644
--- a/src/Neo.Compiler.CSharp/Optimizer/Strategies/Optimizer.cs
+++ b/src/Neo.Compiler.CSharp/Optimizer/Strategies/Optimizer.cs
@@ -64,11 +64,12 @@ public static (NefFile, ContractManifest, JObject?) Optimize(NefFile nef, Contra
             if (!optimizationType.HasFlag(CompilationOptions.OptimizationType.Experimental))
                 return (nef, manifest, debugInfo);  // do nothing
             // TODO in the future: optimize by StrategyAttribute in a loop
-            (nef, manifest, debugInfo) = Reachability.RemoveUnnecessaryJumps(nef, manifest, debugInfo!.Clone() as JObject);
-            (nef, manifest, debugInfo) = Reachability.ReplaceJumpWithRet(nef, manifest, debugInfo!.Clone() as JObject);
-            (nef, manifest, debugInfo) = Reachability.RemoveUncoveredInstructions(nef, manifest, debugInfo!.Clone() as JObject);
-            (nef, manifest, debugInfo) = Reachability.RemoveUnnecessaryJumps(nef, manifest, debugInfo!.Clone() as JObject);
-            (nef, manifest, debugInfo) = Reachability.ReplaceJumpWithRet(nef, manifest, debugInfo!.Clone() as JObject);
+            debugInfo = debugInfo?.Clone() as JObject;  // do not pollute the input when optimization fails
+            (nef, manifest, debugInfo) = Reachability.RemoveUnnecessaryJumps(nef, manifest, debugInfo);
+            (nef, manifest, debugInfo) = Reachability.ReplaceJumpWithRet(nef, manifest, debugInfo);
+            (nef, manifest, debugInfo) = Reachability.RemoveUncoveredInstructions(nef, manifest, debugInfo);
+            (nef, manifest, debugInfo) = Reachability.RemoveUnnecessaryJumps(nef, manifest, debugInfo);
+            (nef, manifest, debugInfo) = Reachability.ReplaceJumpWithRet(nef, manifest, debugInfo);
             return (nef, manifest, debugInfo);
         }
     }