diff --git a/python/test/test_scheme.py b/python/test/test_scheme.py
index 374276a3..7c01e034 100644
--- a/python/test/test_scheme.py
+++ b/python/test/test_scheme.py
@@ -503,11 +503,16 @@ def test_bytes():
         m.f0 = 'abc'
 
     with pytest.raises(ValueError):
-        m.f0 = b'abc'
+        m.f1 = 'abc'
+
+    m.f2 = 'abc'
 
     with pytest.raises(TypeError):
         m.f0 = 1
 
+    with pytest.raises(TypeError):
+        m.f1 = 1
+
 def test_time_point():
     scheme = S.Scheme("""yamls://
 - name: msg
diff --git a/python/tll/scheme.pyx b/python/tll/scheme.pyx
index 59402580..c4f5e37a 100644
--- a/python/tll/scheme.pyx
+++ b/python/tll/scheme.pyx
@@ -742,7 +742,11 @@ cdef class FFixedString(FBase):
 
     cdef pack(FFixedString self, v, dest, tail, int tail_offset): return pack_bytes(v, dest, tail, tail_offset)
     cdef unpack(FFixedString self, src): return unpack_str(src[:self.size])
-    cdef convert(FFixedString self, v): return convert_str(v)
+    cdef convert(FFixedString self, v):
+        v = convert_str(v)
+        if len(v) > self.size:
+            raise ValueError(f"String too long: {len(v)} > {self.size}")
+        return v
     cdef from_string(FFixedString self, str s): return s
 _SUBTYPES[(Type.Bytes, SubType.ByteString)] = FFixedString