diff --git a/src/deflate/deflater.rs b/src/deflate/deflater.rs
index 67e9ba10..93dbd43f 100644
--- a/src/deflate/deflater.rs
+++ b/src/deflate/deflater.rs
@@ -4,7 +4,11 @@ use libdeflater::{CompressionError, CompressionLvl, Compressor};
 
 pub fn deflate(data: &[u8], level: u8, max_size: &AtomicMin) -> PngResult<Vec<u8>> {
     let mut compressor = Compressor::new(CompressionLvl::new(level.into()).unwrap());
-    let capacity = max_size.get().unwrap_or(data.len() / 2);
+    // If adhering to a max_size we need to include at least 9 extra bytes of slack space (as specified in docs).
+    let capacity = max_size
+        .get()
+        .unwrap_or_else(|| compressor.zlib_compress_bound(data.len()))
+        + 9;
     let mut dest = vec![0; capacity];
     let len = compressor
         .zlib_compress(data, &mut dest)