diff --git a/core/src/avm2/bytearray.rs b/core/src/avm2/bytearray.rs
index 79045ab407eb..65cffc15e508 100644
--- a/core/src/avm2/bytearray.rs
+++ b/core/src/avm2/bytearray.rs
@@ -229,11 +229,13 @@ impl ByteArrayStorage {
         let mut buffer = Vec::new();
         let error: Option<Box<dyn std::error::Error>> = match algorithm {
             CompressionAlgorithm::Zlib => {
-                let mut encoder = ZlibEncoder::new(&*self.bytes, Compression::fast());
+                // Note: some content is sensitive to compression type
+                // (as it's visible in the header)
+                let mut encoder = ZlibEncoder::new(&*self.bytes, Compression::best());
                 encoder.read_to_end(&mut buffer).err().map(|e| e.into())
             }
             CompressionAlgorithm::Deflate => {
-                let mut encoder = DeflateEncoder::new(&*self.bytes, Compression::fast());
+                let mut encoder = DeflateEncoder::new(&*self.bytes, Compression::best());
                 encoder.read_to_end(&mut buffer).err().map(|e| e.into())
             }
             #[cfg(feature = "lzma")]
diff --git a/tests/tests/swfs/avm2/bytearray_compress/Test.as b/tests/tests/swfs/avm2/bytearray_compress/Test.as
index cb205e901097..066f9c47192d 100644
--- a/tests/tests/swfs/avm2/bytearray_compress/Test.as
+++ b/tests/tests/swfs/avm2/bytearray_compress/Test.as
@@ -1,3 +1,5 @@
+// compiled with mxmlc
+
 package {
     import flash.utils.ByteArray;
     import flash.utils.Endian;
@@ -36,6 +38,8 @@ package {
 
             ba.uncompress("zlib");
             print("uncompressed (zlib)", ba, true);
+
+            checkZlibPrefix();
         }
 
         function createByteArray(): ByteArray {
@@ -68,5 +72,17 @@ package {
 
             trace("");
         }
+
+        // Issue 13773: gemcraft labirynth
+        // Requires zlib->base64 values to start with "eN".
+        // (which means binary header [120, 218])
+        function checkZlibPrefix() {
+            trace("Checking zlib header:")
+            var ba = createByteArray();
+            ba.compress("zlib");
+            ba.position = 0;
+            trace(ba.readUnsignedByte());
+            trace(ba.readUnsignedByte());
+        }
     }
-}
\ No newline at end of file
+}
diff --git a/tests/tests/swfs/avm2/bytearray_compress/output.txt b/tests/tests/swfs/avm2/bytearray_compress/output.txt
index 54bccdb4e601..d70b0d854347 100644
--- a/tests/tests/swfs/avm2/bytearray_compress/output.txt
+++ b/tests/tests/swfs/avm2/bytearray_compress/output.txt
@@ -26,3 +26,6 @@ uncompressed (zlib) position is at 0
 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99
 
 
+Checking zlib header:
+120
+218
diff --git a/tests/tests/swfs/avm2/bytearray_compress/test.fla b/tests/tests/swfs/avm2/bytearray_compress/test.fla
deleted file mode 100644
index f4c23c5d47e4..000000000000
Binary files a/tests/tests/swfs/avm2/bytearray_compress/test.fla and /dev/null differ
diff --git a/tests/tests/swfs/avm2/bytearray_compress/test.swf b/tests/tests/swfs/avm2/bytearray_compress/test.swf
index 47c62d4649ce..2cb295c70a3f 100644
Binary files a/tests/tests/swfs/avm2/bytearray_compress/test.swf and b/tests/tests/swfs/avm2/bytearray_compress/test.swf differ