diff --git a/libswscale/ops.h b/libswscale/ops.h
index 6392d0ffdf..6fc7e60a02 100644
--- a/libswscale/ops.h
+++ b/libswscale/ops.h
@@ -109,6 +109,10 @@ typedef struct SwsReadWriteOp {
 } SwsReadWriteOp;
 
 typedef struct SwsPackOp {
+    /**
+     * Packed bits are assumed to be LSB-aligned within the underlying
+     * integer type; i.e. (msb) 0 ... X Y Z W (lsb).
+     */
     uint8_t pattern[4]; /* bit depth pattern, from MSB to LSB */
 } SwsPackOp;
 
diff --git a/libswscale/ops_internal.h b/libswscale/ops_internal.h
index 0071f78558..ba4c9b39da 100644
--- a/libswscale/ops_internal.h
+++ b/libswscale/ops_internal.h
@@ -39,7 +39,9 @@ static inline AVRational ff_sws_pixel_expand(SwsPixelType from, SwsPixelType to)
 
 static inline void ff_sws_pack_op_decode(const SwsOp *op, uint64_t mask[4], int shift[4])
 {
-    const int size = ff_sws_pixel_type_size(op->type) * 8;
+    int size = 0;
+    for (int i = 0; i < 4; i++)
+        size += op->pack.pattern[i];
     for (int i = 0; i < 4; i++) {
         const int bits = op->pack.pattern[i];
         mask[i] = (UINT64_C(1) << bits) - 1;
diff --git a/tests/ref/fate/sws-ops-list b/tests/ref/fate/sws-ops-list
index b49f944794..a7d6149d8b 100644
--- a/tests/ref/fate/sws-ops-list
+++ b/tests/ref/fate/sws-ops-list
@@ -1 +1 @@
-e910ff7ceaeb64bfdbac3f652b67403f
+ef1dd10af970984495f6008e43d0fe1b