diff --git a/lib/system/strs_v2.nim b/lib/system/strs_v2.nim
index c77ed6ad1e..95e76b1f8f 100644
--- a/lib/system/strs_v2.nim
+++ b/lib/system/strs_v2.nim
@@ -112,9 +112,10 @@ proc nimToCStringConv(s: NimStringV2): cstring {.compilerproc, nonReloadable, in
 
 proc appendString(dest: var NimStringV2; src: NimStringV2) {.compilerproc, inline.} =
   if src.len > 0:
-    # also copy the \0 terminator:
-    copyMem(unsafeAddr dest.p.data[dest.len], unsafeAddr src.p.data[0], src.len+1)
+    # don't copy the \0 terminator:
+    copyMem(unsafeAddr dest.p.data[dest.len], unsafeAddr src.p.data[0], src.len)
     inc dest.len, src.len
+    dest.p.data[dest.len] = '\0'
 
 proc appendChar(dest: var NimStringV2; c: char) {.compilerproc, inline.} =
   dest.p.data[dest.len] = c
diff --git a/tests/system/t24664.nim b/tests/system/t24664.nim
new file mode 100644
index 0000000000..b38e0b4dfc
--- /dev/null
+++ b/tests/system/t24664.nim
@@ -0,0 +1,14 @@
+discard """
+  output: '''
+TestString123TestString123
+TestString123TestString123
+'''
+"""
+
+proc foostring() = # bug #24664
+  for i in 0..1:
+    var s = "TestString123"
+    s.add s
+    echo s
+
+foostring()
\ No newline at end of file