diff --git a/src/Makefile b/src/Makefile
index ad90451..86dae88 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2288,6 +2288,7 @@
 	test_syn_attr \
 	test_syntax \
 	test_system \
+	test_tab \
 	test_tabline \
 	test_tabpage \
 	test_tagcase \
diff --git a/src/edit.c b/src/edit.c
index d4de825..0519a01 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -9373,7 +9373,7 @@
 	    if (p_sta && in_indent)
 		want_vcol = (want_vcol / curbuf->b_p_sw) * curbuf->b_p_sw;
 	    else
-		want_vcol = tabstop_start(want_vcol, curbuf->b_p_sts,
+		want_vcol = tabstop_start(want_vcol, get_sts_value(),
 						     curbuf->b_p_vsts_array);
 #else
 	    want_vcol = (want_vcol / ts) * ts;
@@ -10203,9 +10203,9 @@
 	temp = (int)curbuf->b_p_sw;
 	temp -= get_nolist_virtcol() % temp;
     }
-    else if (tabstop_count(curbuf->b_p_vsts_array) > 0 || curbuf->b_p_sts > 0)
+    else if (tabstop_count(curbuf->b_p_vsts_array) > 0 || curbuf->b_p_sts != 0)
 	                        /* use 'softtabstop' when set */
-	temp = tabstop_padding(get_nolist_virtcol(), curbuf->b_p_sts,
+	temp = tabstop_padding(get_nolist_virtcol(), get_sts_value(),
 						     curbuf->b_p_vsts_array);
     else			/* otherwise use 'tabstop' */
 	temp = tabstop_padding(get_nolist_virtcol(), curbuf->b_p_ts,
diff --git a/src/option.c b/src/option.c
index 50d42eb..504b92a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -13016,7 +13016,7 @@
 
 /*
  * Return the effective softtabstop value for the current buffer, using the
- * 'tabstop' value when 'softtabstop' is negative.
+ * 'shiftwidth' value when 'softtabstop' is negative.
  */
     long
 get_sts_value(void)
diff --git a/src/testdir/test_tab.vim b/src/testdir/test_tab.vim
index b847dbd..3be3024 100644
--- a/src/testdir/test_tab.vim
+++ b/src/testdir/test_tab.vim
@@ -1,3 +1,4 @@
+" Various tests for inserting a Tab.
 
 " Tests for "r<Tab>" with 'smarttab' and 'expandtab' set/not set.
 " Also test that dv_ works correctly
@@ -43,3 +44,38 @@
   enew!
   set expandtab& smartindent& copyindent& ts& sw& sts&
 endfunc
+
+func Test_softtabstop()
+  new
+  set sts=0 sw=0
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x\tx", getline(1))
+
+  call setline(1, '')
+  set sts=4
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, '')
+  set sts=-1 sw=4
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=0 sw=0 backspace=start
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x      x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=4
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=-1 sw=4
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  set sts=0 sw=0 backspace&
+  bwipe!
+endfunc
diff --git a/src/version.c b/src/version.c
index 9c79148..2e92878 100644
--- a/src/version.c
+++ b/src/version.c
@@ -790,6 +790,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    138,
+/**/
     137,
 /**/
     136,