diff --git a/png.c b/png.c
index c99c40d9c..605f50e9b 100644
--- a/png.c
+++ b/png.c
@@ -2402,12 +2402,14 @@ png_max_pixel_block(png_const_structrp png_ptr)
 void /* PRIVATE */
 png_copy_row(png_const_structrp png_ptr, png_bytep dp, png_const_bytep sp,
    png_uint_32 x/*in INPUT*/, png_uint_32 width/*of INPUT*/,
-   unsigned int pixel_depth, int clear/*clear the final byte*/)
+   unsigned int pixel_depth, int clear/*clear the final byte*/, int x_in_dest)
    /* Copy the row in row_buffer; this is the non-interlaced copy used in both
-    * the read and write code.
+    * the read and write code.  'x_in_dest' specifies whether the 'x' applies to
+    * the destination (sp->dp[x], x_in_dest tru) or the source (sp[x]->dp,
+    * x_in_dest false).
     */
 {
-   png_alloc_size_t cb;
+   png_alloc_size_t cb, offset;
    unsigned int remaining; /* remaining bits in a partial byte */
 
    /* Copy 'cb' pixels, but take care with the last byte because it may
@@ -2419,28 +2421,34 @@ png_copy_row(png_const_structrp png_ptr, png_bytep dp, png_const_bytep sp,
       case 1U: remaining =  width       & 7U;
                debug((x & 7U) == 0U);
                cb = width >> 3;
-               dp += x >> 3;
+               offset = x >> 3;
                break;
       case 2U: remaining = (width << 1) & 6U;
                debug((x & 3U) == 0U);
                cb = width >> 2;
-               dp += x >> 2;
+               offset = x >> 2;
                break;
       case 4U: remaining = (width << 2) & 4U;
                debug((x & 1U) == 0U);
                cb = width >> 1;
-               dp += x >> 1;
+               offset = x >> 1;
                break;
       case 8U: remaining = 0U;
                cb = width;
-               dp += x;
+               offset = x;
                break;
       default: remaining = 0U;
                cb = png_calc_rowbytes(png_ptr, pixel_depth, width);
-               dp += png_calc_rowbytes(png_ptr, pixel_depth, x);
+               offset = png_calc_rowbytes(png_ptr, pixel_depth, x);
                break;
    }
 
+   if (x_in_dest)
+      dp += offset;
+
+   else
+      sp += offset;
+
    memcpy(dp, sp, cb);
 
    if (remaining > 0U)
diff --git a/pngpriv.h b/pngpriv.h
index 10b4bb9c4..a7c222ce8 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -943,13 +943,15 @@ PNG_INTERNAL_FUNCTION(png_alloc_size_t,png_calc_rowbytes,
 PNG_INTERNAL_FUNCTION(unsigned int,png_max_pixel_block,
       (png_const_structrp png_ptr),PNG_EMPTY);
 
-/* Copy the row in row_buffer; this is the non-interlaced copy used in both
- * the read and write code.
+/* Copy the row in row_buffer; this is the non-interlaced copy used in both the
+ * read and write code.  'x_in_dest' specifies whether the 'x' applies to
+ * the destination (sp->dp[x], x_in_dest tru) or the source (sp[x]->dp,
+ * x_in_dest false).
  */
 PNG_INTERNAL_FUNCTION(void, png_copy_row,(png_const_structrp png_ptr,
    png_bytep dp, png_const_bytep sp, png_uint_32 x/*in INPUT*/,
    png_uint_32 width/*of INPUT*/, unsigned int pixel_depth,
-   int clear/*clear the final byte*/),PNG_EMPTY);
+   int clear/*clear the final byte*/, int x_in_dest),PNG_EMPTY);
 
 /* Zlib support */
 #define PNG_UNEXPECTED_ZLIB_RETURN (-7)
diff --git a/pngrutil.c b/pngrutil.c
index 7bb53bafa..42d3ea48d 100644
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -3114,7 +3114,7 @@ copy_row(png_const_structrp png_ptr, png_bytep dp, png_const_bytep sp,
 #     else
          PNG_PIXEL_DEPTH(*png_ptr),
 #     endif
-      clear/*clear partial byte at end of row*/);
+      clear/*clear partial byte at end of row*/, 1/*sp -> dp[x]*/);
 }
 
 #ifdef PNG_READ_INTERLACING_SUPPORTED
diff --git a/pngwrite.c b/pngwrite.c
index 3a1ffbb98..e41dbb873 100644
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -720,7 +720,8 @@ copy_row(png_const_structrp png_ptr, png_bytep row_buffer,
    unsigned int pixel_depth)
 {
    /* Copy row[x..x+count] pixels to row_buffer. */
-   png_copy_row(png_ptr, row_buffer, row, x, count, pixel_depth, 1/*clear*/);
+   png_copy_row(png_ptr, row_buffer, row, x, count, pixel_depth, 1/*clear*/,
+         0/* x_in_dest; row[x]->row_buffer */);
 }
 #endif /* WRITE_TRANSFORMS */