diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index ef21743d89..7040b75c62 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -1500,13 +1500,13 @@ proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc) =
   initLocExpr(p, e.sons[2], b)
   let t = skipTypes(e.sons[1].typ, {tyVar})
   let setLenPattern = if not p.module.compileToCpp:
-      "$1 = ($3) #setLengthSeq(&($1)->Sup, sizeof($4), $2);$n"
+      "$1 = ($3) #setLengthSeqV2(&($1)->Sup, $4, $2);$n"
     else:
-      "$1 = ($3) #setLengthSeq($1, sizeof($4), $2);$n"
+      "$1 = ($3) #setLengthSeqV2($1, $4, $2);$n"
 
   lineCg(p, cpsStmts, setLenPattern, [
       rdLoc(a), rdLoc(b), getTypeDesc(p.module, t),
-      getTypeDesc(p.module, t.skipTypes(abstractInst).sons[0])])
+      genTypeInfo(p.module, t.skipTypes(abstractInst), e.info)])
   gcUsage(e)
 
 proc genSetLengthStr(p: BProc, e: PNode, d: var TLoc) =
diff --git a/lib/system/sysstr.nim b/lib/system/sysstr.nim
index 7640b888ba..42aae66549 100644
--- a/lib/system/sysstr.nim
+++ b/lib/system/sysstr.nim
@@ -226,8 +226,8 @@ proc appendChar(dest: NimString, c: char) {.compilerproc, inline.} =
 
 proc setLengthStr(s: NimString, newLen: int): NimString {.compilerRtl.} =
   var n = max(newLen, 0)
-  if wasMoved(s):
-    result = newOwnedString(s, n)
+  if s == nil:
+    result = mnewString(newLen)
   elif n <= s.space:
     result = s
   else:
@@ -313,6 +313,13 @@ proc setLengthSeq(seq: PGenericSeq, elemSize, newLen: int): PGenericSeq {.
            (newLen*%elemSize)), (result.len-%newLen) *% elemSize)
   result.len = newLen
 
+proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int): PGenericSeq {.
+    compilerRtl.} =
+  if s == nil:
+    result = cast[PGenericSeq](newSeq(typ, newLen))
+  else:
+    result = setLengthSeq(s, typ.base.size, newLen)
+
 # --------------- other string routines ----------------------------------
 proc add*(result: var string; x: int64) =
   let base = result.len