diff --git a/src/Zlib.Shared/Deflate.cs b/src/Zlib.Shared/Deflate.cs
index 9672250..9950b6a 100644
--- a/src/Zlib.Shared/Deflate.cs
+++ b/src/Zlib.Shared/Deflate.cs
@@ -1667,12 +1667,12 @@ internal int SetParams(CompressionLevel level, CompressionStrategy strategy)
 
         internal int SetDictionary(byte[] dictionary)
         {
-            int length = dictionary.Length;
-            int index = 0;
-
             if (dictionary == null || status != INIT_STATE)
                 throw new ZlibException("Stream error.");
 
+            int length = dictionary.Length;
+            int index = 0;
+
             _codec._Adler32 = Adler.Adler32(_codec._Adler32, dictionary, 0, dictionary.Length);
 
             if (length < MIN_MATCH)
@@ -1876,4 +1876,4 @@ internal int Deflate(FlushType flush)
         }
 
     }
-}
\ No newline at end of file
+}