diff --git a/Tests/images/bc5s.png b/Tests/images/bc5s.png
index 657d723053a..5e7a1b95e1b 100644
Binary files a/Tests/images/bc5s.png and b/Tests/images/bc5s.png differ
diff --git a/src/libImaging/BcnDecode.c b/src/libImaging/BcnDecode.c
index 24c800677ca..5e4296eeba1 100644
--- a/src/libImaging/BcnDecode.c
+++ b/src/libImaging/BcnDecode.c
@@ -854,7 +854,7 @@ decode_bcn(
             int sign = strcmp(pixel_format, "BC5S") == 0 ? 1 : 0;
             while (bytes >= 16) {
                 rgba col[16];
-                memset(col, 0, 16 * sizeof(col[0]));
+                memset(col, sign ? 128 : 0, 16 * sizeof(col[0]));
                 decode_bc5_block(col, ptr, sign);
                 put_block(im, state, (const char *)col, sizeof(col[0]), C);
                 ptr += 16;