diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 349c531..de0cc4a 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -431,6 +431,13 @@
 	test_cmp expected actual
 '
 
+test_expect_success 'strbuf_utf8_replace() not producing NUL' '
+	git log --color --pretty="tformat:%<(10,trunc)%s%>>(10,ltrunc)%C(auto)%d" |
+		test_decode_color |
+		nul_to_q >actual &&
+	! grep Q actual
+'
+
 # get new digests (with no abbreviations)
 head1=$(git rev-parse --verify HEAD~0) &&
 head2=$(git rev-parse --verify HEAD~1) &&
diff --git a/utf8.c b/utf8.c
index b30790d..401a6a5 100644
--- a/utf8.c
+++ b/utf8.c
@@ -382,6 +382,9 @@
 			dst += n;
 		}
 
+		if (src >= end)
+			break;
+
 		old = src;
 		n = utf8_width((const char**)&src, NULL);
 		if (!src) 	/* broken utf-8, do nothing */