diff --git a/src/libtakiyasha/qmc/qmcdataciphers.py b/src/libtakiyasha/qmc/qmcdataciphers.py
index 9544558..47315b0 100644
--- a/src/libtakiyasha/qmc/qmcdataciphers.py
+++ b/src/libtakiyasha/qmc/qmcdataciphers.py
@@ -216,6 +216,10 @@ def keystream(self, offset: IntegerLike, length: IntegerLike, /) -> Generator[in
         pending = toint_nofloat(length)
         done = 0
         offset = toint_nofloat(offset)
+        if offset < 0:
+            raise ValueError("first argument 'offset' must be a non-negative integer")
+        if pending < 0:
+            raise ValueError("second argument 'length' must be a non-negative integer")
 
         def mark(p: int) -> None:
             nonlocal pending, done, offset
diff --git a/src/libtakiyasha/stdciphers.py b/src/libtakiyasha/stdciphers.py
index 6429386..5bd37d3 100644
--- a/src/libtakiyasha/stdciphers.py
+++ b/src/libtakiyasha/stdciphers.py
@@ -400,9 +400,11 @@ def __init__(self, key: BytesLike, /) -> None:
 
     def keystream(self, offset: IntegerLike, length: IntegerLike, /) -> Generator[int, None, None]:
         offset = toint_nofloat(offset)
-        if offset < 0:
-            raise ValueError(f"offset must be poritive integer or 0, not {offset}")
         length = toint_nofloat(length)
+        if offset < 0:
+            raise ValueError("first argument 'offset' must be a non-negative integer")
+        if length < 0:
+            raise ValueError("second argument 'length' must be a non-negative integer")
 
         for i in range(offset, offset + length):
             yield self._meta_keystream[i % 256]