diff --git a/mpc.c b/mpc.c
index befd1b5..9aa5a13 100644
--- a/mpc.c
+++ b/mpc.c
@@ -2470,7 +2470,7 @@ mpc_val_t *mpcf_strtriml(mpc_val_t *x) {
 mpc_val_t *mpcf_strtrimr(mpc_val_t *x) {
   char *s = x;
   size_t l = strlen(s);
-  while (isspace((unsigned char)s[l-1])) {
+  while (l > 0 && isspace((unsigned char)s[l-1])) {
     s[l-1] = '\0'; l--;
   }
   return s;