diff --git a/src/edit.c b/src/edit.c
index 9017fd0..0d94095 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -8885,7 +8885,7 @@
 	 */
 	if (	   mode == BACKSPACE_CHAR
 		&& ((p_sta && in_indent)
-		    || (curbuf->b_p_sts != 0
+		    || (get_sts_value() != 0
 			&& curwin->w_cursor.col > 0
 			&& (*(ml_get_cursor() - 1) == TAB
 			    || (*(ml_get_cursor() - 1) == ' '
@@ -8901,7 +8901,7 @@
 	    if (p_sta && in_indent)
 		ts = (int)get_sw_value();
 	    else
-		ts = (int)curbuf->b_p_sts;
+		ts = (int)get_sts_value();
 	    /* Compute the virtual column where we want to be.  Since
 	     * 'showbreak' may get in the way, need to get the last column of
 	     * the previous character. */
@@ -9590,7 +9590,7 @@
      */
     if (!curbuf->b_p_et
 	    && !(p_sta && ind && curbuf->b_p_ts != get_sw_value())
-	    && curbuf->b_p_sts == 0)
+	    && get_sts_value() == 0)
 	return TRUE;
 
     if (stop_arrow() == FAIL)
@@ -9606,8 +9606,8 @@
 
     if (p_sta && ind)		/* insert tab in indent, use 'shiftwidth' */
 	temp = (int)get_sw_value();
-    else if (curbuf->b_p_sts > 0) /* use 'softtabstop' when set */
-	temp = (int)curbuf->b_p_sts;
+    else if (curbuf->b_p_sts != 0) /* use 'softtabstop' when set */
+	temp = (int)get_sts_value();
     else			/* otherwise use 'tabstop' */
 	temp = (int)curbuf->b_p_ts;
     temp -= get_nolist_virtcol() % temp;
@@ -9635,7 +9635,7 @@
     /*
      * When 'expandtab' not set: Replace spaces by TABs where possible.
      */
-    if (!curbuf->b_p_et && (curbuf->b_p_sts || (p_sta && ind)))
+    if (!curbuf->b_p_et && (get_sts_value() || (p_sta && ind)))
     {
 	char_u		*ptr;
 #ifdef FEAT_VREPLACE
diff --git a/src/option.c b/src/option.c
index b6b0bf6..da98071 100644
--- a/src/option.c
+++ b/src/option.c
@@ -8509,11 +8509,6 @@
 	    p_window = Rows - 1;
     }
 
-    if (curbuf->b_p_sts < 0)
-    {
-	errmsg = e_positive;
-	curbuf->b_p_sts = 0;
-    }
     if (curbuf->b_p_ts <= 0)
     {
 	errmsg = e_positive;
@@ -11429,3 +11424,13 @@
 {
     return curbuf->b_p_sw ? curbuf->b_p_sw : curbuf->b_p_ts;
 }
+
+/*
+ * Return the effective softtabstop value for the current buffer, using the
+ * 'tabstop' value when 'softtabstop' is negative.
+ */
+    long
+get_sts_value()
+{
+    return curbuf->b_p_sts < 0 ? get_sw_value() : curbuf->b_p_sts;
+}
diff --git a/src/proto/option.pro b/src/proto/option.pro
index 7d3d8d6..fd71704 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -57,4 +57,5 @@
 int file_ff_differs __ARGS((buf_T *buf, int ignore_empty));
 int check_ff_value __ARGS((char_u *p));
 long get_sw_value __ARGS((void));
+long get_sts_value __ARGS((void));
 /* vim: set ft=c : */
diff --git a/src/version.c b/src/version.c
index e0a6326..0a39ede 100644
--- a/src/version.c
+++ b/src/version.c
@@ -720,6 +720,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    693,
+/**/
     692,
 /**/
     691,