diff --git a/lib/memdebug.c b/lib/memdebug.c
index ef7c0c6ee..28b86b656 100644
--- a/lib/memdebug.c
+++ b/lib/memdebug.c
@@ -72,7 +72,7 @@ void *curl_domalloc(size_t size, int line, char *source)
   return mem;
 }
 
-char *curl_dostrdup(char *str, int line, char *source)
+char *curl_dostrdup(const char *str, int line, char *source)
 {
   char *mem;
   size_t len;
diff --git a/lib/memdebug.h b/lib/memdebug.h
index 58657c35d..fc9a3222a 100644
--- a/lib/memdebug.h
+++ b/lib/memdebug.h
@@ -7,7 +7,7 @@
 void *curl_domalloc(size_t size, int line, char *source);
 void *curl_dorealloc(void *ptr, size_t size, int line, char *source);
 void curl_dofree(void *ptr, int line, char *source);
-char *curl_dostrdup(char *str, int line, char *source);
+char *curl_dostrdup(const char *str, int line, char *source);
 void curl_memdebug(char *logname);
 
 /* file descriptor manipulators */