diff --git a/runtime.c b/runtime.c
index 524e0b5e..bf9d9e76 100644
--- a/runtime.c
+++ b/runtime.c
@@ -1860,8 +1860,12 @@ object Cyc_list2string(void *data, object cont, object lst)
     if (!obj_is_char(cbox)) {
       Cyc_rt_raise2(data, "Expected character but received", cbox);
     }
-    Cyc_utf8_encode_char(cbuf, 5, ch);
-    len += strlen(cbuf);
+    if (!ch) {
+      len++;
+    } else {
+      Cyc_utf8_encode_char(cbuf, 5, ch);
+      len += strlen(cbuf);
+    }
     tmp = cdr(tmp);
   }
 
@@ -1871,8 +1875,12 @@ object Cyc_list2string(void *data, object cont, object lst)
     while ((lst != NULL)) {
       cbox = car(lst);
       ch = obj_obj2char(cbox); // Already validated, can assume chars now
-      Cyc_utf8_encode_char(&(buf[i]), 5, ch);
-      i += strlen(buf+i);
+      if (!ch) {
+        i++;
+      } else {
+        Cyc_utf8_encode_char(&(buf[i]), 5, ch);
+        i += strlen(buf+i);
+      }
       lst = cdr(lst);
     }
     buf[i] = '\0';
@@ -2130,8 +2138,12 @@ object Cyc_string_set(void *data, object str, object k, object chr)
   }
 
   input_char = obj_obj2char(chr);
-  Cyc_utf8_encode_char(buf, 5, input_char);
-  buf_len = strlen(buf);
+  if (!input_char) {
+    buf_len = 1;
+  } else {
+    Cyc_utf8_encode_char(buf, 5, input_char);
+    buf_len = strlen(buf);
+  }
 
   raw = string_str(str);
   idx = unbox_number(k);
diff --git a/scheme/base.sld b/scheme/base.sld
index 5ab42fd0..c13cde79 100644
--- a/scheme/base.sld
+++ b/scheme/base.sld
@@ -958,8 +958,12 @@
           Cyc_rt_raise2(data, \"Expected character buf received\", fill);
         }
         c = obj_obj2char(fill);
-        Cyc_utf8_encode_char(ch_buf, 5, c);
-        buflen = strlen(ch_buf);
+        if (!c) {
+          buflen = 1;
+        } else {
+          Cyc_utf8_encode_char(ch_buf, 5, c);
+          buflen = strlen(ch_buf);
+        }
         num_cp = obj_obj2int(count);
         len = num_cp * buflen;
         if (len >= MAX_STACK_OBJ) {