diff --git a/string.c b/string.c
index 6d5bccc..0e4fb86 100644
--- a/string.c
+++ b/string.c
@@ -108,7 +108,7 @@ const char *strstr(const char *haystack, const char *needle)
 {
     int npos = 0; /* position within needle */
     int nlen = strlen(needle);
-    char *start;
+    const char *start;
 
     while (*haystack != '\0') {
         if (npos == 0) start = haystack;