diff --git a/scheme/base.sld b/scheme/base.sld
index 0a63fb55..f5451340 100644
--- a/scheme/base.sld
+++ b/scheme/base.sld
@@ -951,11 +951,16 @@
       "(void *data, int argc, closure _, object k, object count, object fill)"
       " object s = NULL;
         char ch_buf[5];
+        char_type c;
+        int buflen, num_cp, len;
         Cyc_check_int(data, count);
-        char_type c = obj_obj2char(fill);
-        Cyc_utf8_encode_char(ch_buf, 5, c);
-        int num_cp = obj_obj2int(count);
-        int len = num_cp * strlen(ch_buf);
+        if (!obj_is_char(fill)) {
+          Cyc_rt_raise2(data, \"Expected character buf received\", fill);
+        }
+        c = obj_obj2char(fill);
+        buflen = Cyc_utf8_encode_char(ch_buf, 5, c);
+        num_cp = obj_obj2int(count);
+        len = num_cp * buflen;
         if (len >= MAX_STACK_OBJ) {
           int heap_grown;
           s = gc_alloc(((gc_thread_data *)data)->heap, 
@@ -982,7 +987,7 @@
           memset(((string_type *)s)->str, ch_buf[0], len);
         } else {
           char *buf = ((string_type *)s)->str;
-          int bi, si, slen = strlen(ch_buf);
+          int bi, si, slen = buflen;
           for (bi = 0, si = 0; bi < len; bi++, si++) {
             buf[bi] = ch_buf[si % slen];
           }