diff --git a/src/ops.c b/src/ops.c
index 88992b6..80e0ea1 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -527,24 +527,8 @@ block_insert(
 	}
 
 	if (has_mbyte && spaces > 0)
-	{
-	    int off;
-
-	    // Avoid starting halfway a multi-byte character.
-	    if (b_insert)
-	    {
-		off = (*mb_head_off)(oldp, oldp + offset + spaces);
-		spaces -= off;
-		count -= off;
-	    }
-	    else
-	    {
-		// spaces fill the gap, the character that's at the edge moves
-		// right
-		off = (*mb_head_off)(oldp, oldp + offset);
-		offset -= off;
-	    }
-	}
+	    // avoid copying part of a multi-byte character
+	    offset -= (*mb_head_off)(oldp, oldp + offset);
 
 	// Make sure the allocated size matches what is actually copied below.
 	newp = alloc(STRLEN(oldp) + spaces + s_len
diff --git a/src/testdir/test_utf8.vim b/src/testdir/test_utf8.vim
index 5454e43..bedec20 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -7,7 +7,7 @@ func Test_visual_block_insert()
   new
   call setline(1, ["aaa", "あああ", "bbb"])
   exe ":norm! gg0l\<C-V>jjIx\<Esc>"
-  call assert_equal(['axaa', 'xあああ', 'bxbb'], getline(1, '$'))
+  call assert_equal(['axaa', ' xあああ', 'bxbb'], getline(1, '$'))
   bwipeout!
 endfunc
 
diff --git a/src/testdir/test_visual.vim b/src/testdir/test_visual.vim
index dc8e376..8de9e3d 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -976,4 +976,13 @@ func Test_visual_block_append_invalid_char()
   bwipe!
 endfunc
 
+func Test_visual_block_insert_round_off()
+  new
+  " The number of characters are tuned to fill a 4096 byte allocated block,
+  " so that valgrind reports going over the end.
+  call setline(1, ['xxxxx', repeat('0', 1350), "\t", repeat('x', 60)])
+  exe "normal gg0\<C-V>GI" .. repeat('0', 1320) .. "\<Esc>"
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab