diff --git a/cfg.h b/cfg.h
index 85ac215..98c5982 100644
--- a/cfg.h
+++ b/cfg.h
@@ -59,6 +59,7 @@ char* string_cstr(string* s) {
 }
 
 string string_from_cstr(char* c) {
+  assert(c != NULL);
   size_t l = strlen(c);
   string s = new_string_sized(l);
   s.len = l;
@@ -72,6 +73,7 @@ void string_grow_min(string* s, size_t min_growth) {
 }
 
 void string_cappend(string* s, char* c) {
+  assert(c != NULL);
   size_t l = strlen(c);
   if (s->cap < s->len + l) string_grow_min(s, l);
   memcpy(s->str+s->len, c, l);