diff --git a/src/spell.c b/src/spell.c
index a4c6811..ce887f7 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -12182,7 +12182,9 @@
 	    {
 		n = mb_cptr2len(p);
 		c = mb_ptr2char(p);
-		if (!soundfold && !spell_iswordp(p + n, curbuf))
+		if (p[n] == NUL)
+		    c2 = NUL;
+		else if (!soundfold && !spell_iswordp(p + n, curbuf))
 		    c2 = c; /* don't swap non-word char */
 		else
 		    c2 = mb_ptr2char(p + n);
@@ -12190,12 +12192,21 @@
 	    else
 #endif
 	    {
-		if (!soundfold && !spell_iswordp(p + 1, curbuf))
+		if (p[1] == NUL)
+		    c2 = NUL;
+		else if (!soundfold && !spell_iswordp(p + 1, curbuf))
 		    c2 = c; /* don't swap non-word char */
 		else
 		    c2 = p[1];
 	    }
 
+	    /* When the second character is NUL we can't swap. */
+	    if (c2 == NUL)
+	    {
+		sp->ts_state = STATE_REP_INI;
+		break;
+	    }
+
 	    /* When characters are identical, swap won't do anything.
 	     * Also get here if the second char is not a word character. */
 	    if (c == c2)
diff --git a/src/version.c b/src/version.c
index 95f2a4f..a8edba4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -667,6 +667,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    51,
+/**/
     50,
 /**/
     49,