diff -r 1ba4ec2e7a89 -r ebdce4bf262b tests/test_string.cpp
--- a/tests/test_string.cpp	Sat Apr 22 12:29:00 2023 +0200
+++ b/tests/test_string.cpp	Sat Apr 22 13:06:18 2023 +0200
@@ -273,6 +273,13 @@
     EXPECT_EQ(cx_strcmp(cx_strcast(t5), CX_STR("561234")), 0);
     EXPECT_ZERO_TERMINATED(t5);
     cx_strfree(&t5);
+
+    // use an initial string
+    cxmutstr t6 = cx_strdup(CX_STR("Hello"));
+    t6 = cx_strcat_m(t6, 2, CX_STR(", "), CX_STR("World!"));
+    EXPECT_EQ(cx_strcmp(cx_strcast(t6), CX_STR("Hello, World!")), 0);
+    EXPECT_ZERO_TERMINATED(t6);
+    cx_strfree(&t6);
 }
 
 TEST(String, strsplit) {