diff --git a/compiler/int128.nim b/compiler/int128.nim
index 74e581cd51..cc253fb682 100644
--- a/compiler/int128.nim
+++ b/compiler/int128.nim
@@ -460,7 +460,9 @@ proc addInt128*(result: var string; value: Int128) =
     var i = initialSize
     var j = high(result)
     while i < j:
-      swap(result[i], result[j])
+      let tmp = result[i]
+      result[i] = result[j]
+      result[j] = tmp
       i += 1
       j -= 1
 
diff --git a/lib/system/strs_v3.nim b/lib/system/strs_v3.nim
index ff22e24ecc..81c52d3134 100644
--- a/lib/system/strs_v3.nim
+++ b/lib/system/strs_v3.nim
@@ -574,6 +574,7 @@ proc setLengthStrV2(s: var SmallString; newLen: int) {.compilerRtl.} =
 
 proc nimAsgnStrV2(a: var SmallString; b: SmallString) {.compilerRtl, inline.} =
   if ssLen(b) <= PayloadSize:
+    nimDestroyStrV1(a)  # free any existing heap block before overwriting
     copyMem(addr a, unsafeAddr b, sizeof(SmallString))
   else:
     if addr(a) == unsafeAddr(b): return