diff --git a/src/ops.c b/src/ops.c
index 6a37844..a996802 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -536,24 +536,29 @@
 	    if (b_insert)
 	    {
 		off = (*mb_head_off)(oldp, oldp + offset + spaces);
+		spaces -= off;
+		count -= off;
 	    }
 	    else
 	    {
-		off = (*mb_off_next)(oldp, oldp + offset);
-		offset += off;
+		// spaces fill the gap, the character that's at the edge moves
+		// right
+		off = (*mb_head_off)(oldp, oldp + offset);
+		offset -= off;
 	    }
-	    spaces -= off;
-	    count -= off;
 	}
 	if (spaces < 0)  // can happen when the cursor was moved
 	    spaces = 0;
 
-	newp = alloc(STRLEN(oldp) + s_len + count + 1);
+	// Make sure the allocated size matches what is actually copied below.
+	newp = alloc(STRLEN(oldp) + spaces + s_len
+		    + (spaces > 0 && !bdp->is_short ? ts_val - spaces : 0)
+								  + count + 1);
 	if (newp == NULL)
 	    continue;
 
 	// copy up to shifted part
-	mch_memmove(newp, oldp, (size_t)(offset));
+	mch_memmove(newp, oldp, (size_t)offset);
 	oldp += offset;
 
 	// insert pre-padding
@@ -564,14 +569,21 @@
 	mch_memmove(newp + startcol, s, (size_t)s_len);
 	offset += s_len;
 
-	if (spaces && !bdp->is_short)
+	if (spaces > 0 && !bdp->is_short)
 	{
-	    // insert post-padding
-	    vim_memset(newp + offset + spaces, ' ', (size_t)(ts_val - spaces));
-	    // We're splitting a TAB, don't copy it.
-	    oldp++;
-	    // We allowed for that TAB, remember this now
-	    count++;
+	    if (*oldp == TAB)
+	    {
+		// insert post-padding
+		vim_memset(newp + offset + spaces, ' ',
+						    (size_t)(ts_val - spaces));
+		// we're splitting a TAB, don't copy it
+		oldp++;
+		// We allowed for that TAB, remember this now
+		count++;
+	    }
+	    else
+		// Not a TAB, no extra spaces
+		count = spaces;
 	}
 
 	if (spaces > 0)
@@ -1598,7 +1610,7 @@
 		    oap->start_vcol = t;
 		}
 		else if (oap->op_type == OP_APPEND
-			&& oap->end.col + oap->end.coladd
+			&& oap->start.col + oap->start.coladd
 				>= curbuf->b_op_start_orig.col
 					      + curbuf->b_op_start_orig.coladd)
 		{
diff --git a/src/testdir/test_visual.vim b/src/testdir/test_visual.vim
index e40be5d..9ca0fa9 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -1278,6 +1278,15 @@
   au! BufNew
 endfunc
 
+func Test_visual_block_append_invalid_char()
+  " this was going over the end of the line
+  new
+  call setline(1, ['	   let xxx', 'xxxxx', 'xxxxxxxxxxx'])
+  exe "normal 0\<C-V>jjA-\<Esc>"
+  call assert_equal(['	-   let xxx', 'xxxxx   -', 'xxxxxxxx-xxx'], getline(1, 3))
+  bwipe!
+endfunc
+
 func Test_visual_reselect_with_count()
   " this was causing an illegal memory access
   let lines =<< trim END
diff --git a/src/version.c b/src/version.c
index 01b8e5d..6fda77e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4120,
+/**/
     4119,
 /**/
     4118,