diff --git a/src/option.c b/src/option.c
index 97e45cd..b6b0bf6 100644
--- a/src/option.c
+++ b/src/option.c
@@ -8125,7 +8125,7 @@
     need_mouse_correct = TRUE;
 #endif
 
-    if (curbuf->b_p_sw <= 0)
+    if (curbuf->b_p_sw < 0)
     {
 	errmsg = e_positive;
 	curbuf->b_p_sw = curbuf->b_p_ts;
@@ -11419,3 +11419,13 @@
 {
     return check_opt_strings(p, p_ff_values, FALSE);
 }
+
+/*
+ * Return the effective shiftwidth value for current buffer, using the
+ * 'tabstop' value when 'shiftwidth' is zero.
+ */
+    long
+get_sw_value()
+{
+    return curbuf->b_p_sw ? curbuf->b_p_sw : curbuf->b_p_ts;
+}