diff --git a/lib/system/sysstr.nim b/lib/system/sysstr.nim
index 9ecdffb669..c879558dd0 100644
--- a/lib/system/sysstr.nim
+++ b/lib/system/sysstr.nim
@@ -264,7 +264,7 @@ proc setLengthStrUninit(s: var string, newlen: Natural) {.nodestroy.} =
       str.data[n] = '\0'
       str.len = n
       s = cast[string](str)
-    elif n < s.len:
+    elif n != s.len:
       str.data[n] = '\0'
       str.len = n
     else: return
diff --git a/tests/stdlib/tstring.nim b/tests/stdlib/tstring.nim
index 724eef4314..fad3865085 100644
--- a/tests/stdlib/tstring.nim
+++ b/tests/stdlib/tstring.nim
@@ -176,6 +176,13 @@ proc main() =
         result.add char(ord('a') + i mod 26)
 
     proc checkSetLenUninit(oldLen, newLen: int; cmpAfter = -1) =
+      ## Verifies `setLenUninit`:
+      ## - preserves the existing prefix
+      ## - updates the string length
+      ## - keeps internal null termination valid for both shrink and growth
+      ##
+      ## `cmpAfter` is used for layouts where trailing zeroed padding affects
+      ## string comparison semantics after the resize.
       var s = makeStr(oldLen)
       let prefixLen = min(oldLen, newLen)
       let prefix = makeStr(prefixLen)
@@ -203,9 +210,18 @@ proc main() =
 
     block setLenUninit:
       # Shared baseline for both SSO and V2: noop, shrink, grow.
-      checkSetLenUninit(numbers.len, numbers.len)
-      checkSetLenUninit(numbers.len, 5)
-      checkSetLenUninit(numbers.len, 11)
+      checkSetLenUninit(10, 10)
+      checkSetLenUninit(10, 5)
+      checkSetLenUninit(10, 11)
+
+      block growingWithinBiggerCapacity:
+        # Strings can reserve spare capacity even for short strings.
+        # Growing within that capacity must still update len and the trailing zero.
+        var s = newStringOfCap(10)
+        s.add("abc")
+        s.setLenUninit(6)
+        s.checkStrInternals(6)
+        doAssert s[0..2] == "abc"
 
       when hasNativeSso:
         const