diff --git a/nimbus/vm/computation.nim b/nimbus/vm/computation.nim
index 0d8accd71..86efc4a63 100644
--- a/nimbus/vm/computation.nim
+++ b/nimbus/vm/computation.nim
@@ -344,3 +344,6 @@ proc traceOpCodeEnded*(c: BaseComputation, op: Op, lastIndex: int) =
 
 proc traceError*(c: BaseComputation) =
   c.vmState.tracer.traceError(c)
+
+proc prepareTracer*(c: BaseComputation) =
+  c.vmState.tracer.prepare(c.msg.depth)
diff --git a/nimbus/vm/interpreter_dispatch.nim b/nimbus/vm/interpreter_dispatch.nim
index 08350897c..848849cad 100644
--- a/nimbus/vm/interpreter_dispatch.nim
+++ b/nimbus/vm/interpreter_dispatch.nim
@@ -228,6 +228,7 @@ proc opTableToCaseStmt(opTable: array[Op, NimNode], computation: NimNode): NimNo
 
   # Wrap the case statement in while true + computed goto
   result = quote do:
+    `computation`.prepareTracer()
     var `instr` = `computation`.code.next()
     while true:
       {.computedGoto.}
diff --git a/nimbus/vm/transaction_tracer.nim b/nimbus/vm/transaction_tracer.nim
index 0534f48f4..f863f9921 100644
--- a/nimbus/vm/transaction_tracer.nim
+++ b/nimbus/vm/transaction_tracer.nim
@@ -24,13 +24,16 @@ proc initTracer*(tracer: var TransactionTracer, flags: set[TracerFlags] = {}) =
   tracer.accounts = initSet[EthAddress]()
   tracer.storageKeys = @[]
 
-proc rememberStorageKey(tracer: var TransactionTracer, compDepth: int, key: Uint256) =
+proc prepare*(tracer: var TransactionTracer, compDepth: int) =
   if compDepth >= tracer.storageKeys.len:
     let prevLen = tracer.storageKeys.len
     tracer.storageKeys.setLen(compDepth + 1)
-    for i in prevLen ..< tracer.storageKeys.len:
+    for i in prevLen ..< tracer.storageKeys.len - 1:
       tracer.storageKeys[i] = initSet[Uint256]()
 
+  tracer.storageKeys[compDepth] = initSet[Uint256]()
+
+proc rememberStorageKey(tracer: var TransactionTracer, compDepth: int, key: Uint256) =
   tracer.storageKeys[compDepth].incl key
 
 iterator storage(tracer: TransactionTracer, compDepth: int): Uint256 =