1 files changed, 4 insertions, 1 deletions
diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index 4ded438f52..ef9f0ca408 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -127,6 +127,7 @@ int setmark_pos(int c, pos_T *pos, int fnum)
     return OK;
   }
   if (isupper(c)) {
+    assert(c >= 'A' && c <= 'Z');
     i = c - 'A';
     namedfm[i].fmark.mark = *pos;
     namedfm[i].fmark.fnum = fnum;
@@ -1219,8 +1220,10 @@ int read_viminfo_filemark(vir_T *virp, int force)
       }
     } else if (VIM_ISDIGIT(*str))
       fm = &namedfm[*str - '0' + NMARKS];
-    else
+    else {  // is uppercase
+      assert(*str >= 'A' && *str <= 'Z');
       fm = &namedfm[*str - 'A'];
+    }
     if (fm != NULL && (fm->fmark.mark.lnum == 0 || force)) {
       str = skipwhite(str + 1);
       fm->fmark.mark.lnum = getdigits(&str);