diff --git a/termkey.c b/termkey.c
index 59b1859..c4ee594 100644
--- a/termkey.c
+++ b/termkey.c
@@ -1132,7 +1132,7 @@ char *termkey_strpkey(TermKey *tk, const char *str, TermKeyKey *key, TermKeyForm
     key->type = TERMKEY_TYPE_KEYSYM;
     str = endstr;
   }
-  else if(sscanf(str, "F%d%n", &key->code.number, &nbytes) == 1) {
+  else if(sscanf(str, "F%d%zn", &key->code.number, &nbytes) == 1) {
     key->type = TERMKEY_TYPE_FUNCTION;
     str += nbytes;
   }