diff --git a/m-string.h b/m-string.h
index 929c61a6..de4c61b3 100644
--- a/m-string.h
+++ b/m-string.h
@@ -280,8 +280,9 @@ string_cat(string_t v, const string_t v2)
   if (M_LIKELY (size > 0)) {
     stringi_fit2size(v, v->size + size + 1);
     assert (v2->ptr != NULL);
-    memcpy(&v->ptr[v->size], v2->ptr, size + 1);
+    memcpy(&v->ptr[v->size], v2->ptr, size);
     v->size += size;
+    v->ptr[v->size] = 0;
   }
   STRING_CONTRACT (v);
 }
diff --git a/tests/test-mstring.c b/tests/test-mstring.c
index 1a858f6c..9f0dc355 100644
--- a/tests/test-mstring.c
+++ b/tests/test-mstring.c
@@ -202,7 +202,10 @@ int main(void)
   string_swap (s1, s2);
   assert(string_equal_str_p (s1, "QWERTY"));
   assert(string_equal_str_p (s2, "AZERTY"));
-  
+
+  string_cat(s1, s1);
+  assert (string_equal_str_p(s1, "QWERTYQWERTY"));
+
   string_clear (s2);
   string_clear (s1);
   exit(0);