diff --git a/src/openvpn/dhcp.c b/src/openvpn/dhcp.c
index a54ab3f..5cdcfcf 100644
--- a/src/openvpn/dhcp.c
+++ b/src/openvpn/dhcp.c
@@ -277,7 +277,9 @@
     {
         const char *ptr = str_array[i];
 
-        if (strlen(ptr) + len + 1 > sizeof(tmp_buf))
+        /* Each entry consumes strlen(ptr) + 2 bytes: one leading label length
+         * byte and one trailing NUL. */
+        if (strlen(ptr) + len + 2 > sizeof(tmp_buf))
         {
             *error = true;
             msg(M_WARN, "write_dhcp_search_str: temp buffer overflow building DHCP options");
diff --git a/tests/unit_tests/openvpn/test_dhcp.c b/tests/unit_tests/openvpn/test_dhcp.c
index 104fc9a..3a84e1e 100644
--- a/tests/unit_tests/openvpn/test_dhcp.c
+++ b/tests/unit_tests/openvpn/test_dhcp.c
@@ -120,6 +120,24 @@
     assert_memory_equal(BPTR(&out_buf), output_5, sizeof(output_5));
     assert_false(error);
 
+    /* Several entries whose accumulated length lands exactly on the guard
+     * boundary. Each entry consumes strlen()+2 bytes of tmp_buf (one length
+     * prefix plus one trailing NUL), but the guard only accounts for
+     * strlen()+1, so the last entry writes one byte past tmp_buf[256].
+     * Sizes: 4 x 50 leaves len == 208, the final 47 makes
+     * 47 + 208 + 1 == 256, which the guard still accepts. */
+#define D50 "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+#define D47 "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+    const char *overflow_list[] = { D50, D50, D50, D50, D47 };
+    assert_int_equal(strlen(D50), 50);
+    assert_int_equal(strlen(D47), 47);
+    buf_clear(&out_buf);
+    write_dhcp_search_str(&out_buf, DHCP_DOMAIN_SEARCH, overflow_list, 5, &error);
+    /* total is 257 > 255, so the option must be rejected -- the point of this
+     * case is that tmp_buf must not be written out of bounds on the way. */
+    assert_true(error);
+    error = false;
+
     gc_free(&gc);
 }