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 dfd4b3e07a4..5e4296eeba1 100644
--- a/src/libImaging/BcnDecode.c
+++ b/src/libImaging/BcnDecode.c
@@ -850,10 +850,12 @@ decode_bcn(
         DECODE_LOOP(3, 16, rgba);
         DECODE_LOOP(4, 8, lum);
         case 5:
+        {
+            int sign = strcmp(pixel_format, "BC5S") == 0 ? 1 : 0;
             while (bytes >= 16) {
                 rgba col[16];
-                memset(col, 0, 16 * sizeof(col[0]));
-                decode_bc5_block(col, ptr, strcmp(pixel_format, "BC5S") == 0 ? 1 : 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;
                 bytes -= 16;
@@ -862,10 +864,13 @@ decode_bcn(
                 }
             }
             break;
+        }
         case 6:
+        {
+            int sign = strcmp(pixel_format, "BC6HS") == 0 ? 1 : 0;
             while (bytes >= 16) {
                 rgba col[16];
-                decode_bc6_block(col, ptr, strcmp(pixel_format, "BC6HS") == 0 ? 1 : 0);
+                decode_bc6_block(col, ptr, sign);
                 put_block(im, state, (const char *)col, sizeof(col[0]), C);
                 ptr += 16;
                 bytes -= 16;
@@ -874,6 +879,7 @@ decode_bcn(
                 }
             }
             break;
+        }
         DECODE_LOOP(7, 16, rgba);
 #undef DECODE_LOOP
     }