diff --git a/src/gdhelpers.c b/src/gdhelpers.c
index de7a0a3..302dc18 100644
--- a/src/gdhelpers.c
+++ b/src/gdhelpers.c
@@ -15,13 +15,13 @@
 #define SEP_TEST (separators[*((unsigned char *) s)])
 
 char *
-gd_strtok_r (char *s, char *sep, char **state)
+gd_strtok_r(char *s, const char *sep, char **state)
 {
 	char separators[256];
 	char *result = 0;
 	memset (separators, 0, sizeof (separators));
 	while (*sep) {
-		separators[*((unsigned char *) sep)] = 1;
+		separators[*((const unsigned char *) sep)] = 1;
 		sep++;
 	}
 	if (!s) {
diff --git a/src/gdhelpers.h b/src/gdhelpers.h
index 9c7ef10..687e156 100644
--- a/src/gdhelpers.h
+++ b/src/gdhelpers.h
@@ -14,7 +14,7 @@ extern "C" {
 
 	/* TBB: strtok_r is not universal; provide an implementation of it. */
 
-	char * gd_strtok_r (char *s, char *sep, char **state);
+	char *gd_strtok_r(char *s, const char *sep, char **state);
 
 	/* These functions wrap memory management. gdFree is
 		in gd.h, where callers can utilize it to correctly