diff --git a/src/option.c b/src/option.c
index 8e91fae..5247893 100644
--- a/src/option.c
+++ b/src/option.c
@@ -7479,6 +7479,19 @@
     }
 #endif
 
+#ifdef FEAT_TERMINAL
+    /* 'termsize' */
+    else if (varp == &curwin->w_p_tms)
+    {
+	if (*curwin->w_p_tms != NUL)
+	{
+	    p = skipdigits(curwin->w_p_tms);
+	    if (p == curwin->w_p_tms || *p != 'x' || *skipdigits(p + 1) != NUL)
+		errmsg = e_invarg;
+	}
+    }
+#endif
+
     /* Options that are a list of flags. */
     else
     {
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 019c757..128a918 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -124,6 +124,7 @@
       \ 'switchbuf': [['', 'useopen', 'split,newtab'], ['xxx']],
       \ 'tagcase': [['smart', 'match'], ['', 'xxx', 'smart,match']],
       \ 'term': [[], []],
+      \ 'termsize': [['', '24x80', '0x80', '32x0', '0x0'], ['xxx', '80', '8ax9', '24x80b']],
       \ 'toolbar': [['', 'icons', 'text'], ['xxx']],
       \ 'toolbariconsize': [['', 'tiny', 'huge'], ['xxx']],
       \ 'ttymouse': [['', 'xterm'], ['xxx']],
diff --git a/src/version.c b/src/version.c
index b678ed9..da56b9a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    743,
+/**/
     742,
 /**/
     741,