diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index dbbfb8d9..21b204c5 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -716,7 +716,7 @@ void Cyc_set_globals_changed(gc_thread_data *thd);
 #define CYC_UTF8_ACCEPT 0
 #define CYC_UTF8_REJECT 1
 uint32_t Cyc_utf8_decode(uint32_t* state, uint32_t* codep, uint32_t byte);
-int Cyc_utf8_count_code_points(uint8_t* s, size_t* count);
+int Cyc_utf8_count_code_points(uint8_t* s);
 uint32_t Cyc_utf8_validate_stream(uint32_t *state, char *str, size_t len); 
 uint32_t Cyc_utf8_validate(char *str, size_t len);
 /**@}*/
diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index f6fb2b1e..776aada6 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -764,17 +764,20 @@ typedef struct {
   cs.num_cp = length; \
   cs.str = s; }
 
-///** Create a new string in the nursery */
-//#define make_string(cs, s) string_type cs; \
-//{ int len = strlen(s); \
-//  cs.hdr.mark = gc_color_red; \
-//  cs.hdr.grayed = 0; \
-//  cs.tag = string_tag; \
-//  cs.num_cp = len; \
-//  cs.len = len; \
-//  cs.str = alloca(sizeof(char) * (len + 1)); \
-//  memcpy(cs.str, s, len + 1);}
-//
+/** Create a new string in the nursery */
+#define make_utf8_string(data, cs, s) string_type cs; \
+{ int len = strlen(s); \
+  cs.hdr.mark = gc_color_red; \
+  cs.hdr.grayed = 0; \
+  cs.tag = string_tag; \
+  cs.num_cp = Cyc_utf8_count_code_points(s); \
+  if (cs.num_cp < 0) { \
+    Cyc_rt_raise_msg(data, "Invalid UTF-8 characters in string"); \
+  } \
+  cs.len = len; \
+  cs.str = alloca(sizeof(char) * (len + 1)); \
+  memcpy(cs.str, s, len + 1);}
+
 ///** 
 // * Create a new string with the given length 
 // * (so it does not need to be computed) 
diff --git a/runtime.c b/runtime.c
index a0662e81..ef2bafb0 100644
--- a/runtime.c
+++ b/runtime.c
@@ -6403,15 +6403,18 @@ uint32_t Cyc_utf8_decode(uint32_t* state, uint32_t* codep, uint32_t byte) {
  * Count the number of code points in a string.
  * Based on example code from Bjoern Hoehrmann.
  */
-int Cyc_utf8_count_code_points(uint8_t* s, size_t* count) {
+int Cyc_utf8_count_code_points(uint8_t* s) {
   uint32_t codepoint;
   uint32_t state = 0;
+  int count;
 
-  for (*count = 0; *s; ++s)
+  for (count = 0; *s; ++s)
     if (!Cyc_utf8_decode(&state, &codepoint, *s))
-      *count += 1;
+      count += 1;
 
-  return state != CYC_UTF8_ACCEPT;
+  if (state != CYC_UTF8_ACCEPT)
+    return -1;
+  return count;
 }
 
 // TODO: index into X codepoint in a string