diff --git a/upb/bytestream.c b/upb/bytestream.c
index 4f6e4b104c..812e55226d 100644
--- a/upb/bytestream.c
+++ b/upb/bytestream.c
@@ -32,6 +32,8 @@ upb_byteregion *upb_byteregion_newl(const void *str, size_t len) {
   memcpy(ptr, str, len);
   ptr[len] = '\0';
   upb_stringsrc_reset(src, ptr, len);
+  upb_byteregion_fetch(upb_stringsrc_allbytes(src));
+  assert(len == upb_byteregion_available(upb_stringsrc_allbytes(src), 0));
   return upb_stringsrc_allbytes(src);
 }
 
@@ -241,9 +243,10 @@ upb_bytesink* upb_stdio_bytesink(upb_stdio *stdio) { return &stdio->sink; }
 
 upb_bytesuccess_t upb_stringsrc_fetch(void *_src, uint64_t ofs, size_t *read) {
   upb_stringsrc *src = _src;
-  assert(ofs < src->len);
+  assert(ofs <= src->len);
   if (ofs == src->len) {
     upb_status_seteof(&src->bytesrc.status);
+    *read = 0;
     return UPB_BYTE_EOF;
   }
   *read = src->len - ofs;
diff --git a/upb/def.c b/upb/def.c
index 246e9bb7a0..5ac3498868 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -235,6 +235,7 @@ upb_fielddef *upb_fielddef_new() {
 }
 
 static void upb_fielddef_init_default(upb_fielddef *f) {
+  f->default_is_string = false;
   switch (upb_fielddef_type(f)) {
     case UPB_TYPE(DOUBLE): upb_value_setdouble(&f->defaultval, 0); break;
     case UPB_TYPE(FLOAT): upb_value_setfloat(&f->defaultval, 0); break;
@@ -252,15 +253,16 @@ static void upb_fielddef_init_default(upb_fielddef *f) {
     case UPB_TYPE(BOOL): upb_value_setbool(&f->defaultval, false); break;
     case UPB_TYPE(STRING):
     case UPB_TYPE(BYTES):
-        upb_value_setbyteregion(&f->defaultval, upb_byteregion_new("")); break;
+      f->default_is_string = true;
+      upb_value_setbyteregion(&f->defaultval, upb_byteregion_new(""));
+      break;
     case UPB_TYPE(GROUP):
     case UPB_TYPE(MESSAGE): upb_value_setptr(&f->defaultval, NULL); break;
   }
-  f->default_is_symbolic = false;
 }
 
 static void upb_fielddef_uninit_default(upb_fielddef *f) {
-  if (upb_isstring(f) || f->default_is_symbolic) {
+  if (f->default_is_string) {
     upb_byteregion_free(upb_value_getbyteregion(f->defaultval));
   }
 }
@@ -323,7 +325,7 @@ static bool upb_fielddef_resolve(upb_fielddef *f, upb_def *def, upb_status *s) {
   assert(upb_dyncast_unresolveddef(f->def));
   upb_def_unref(f->def);
   f->def = def;
-  if (f->type == UPB_TYPE(ENUM) && f->default_is_symbolic) {
+  if (f->type == UPB_TYPE(ENUM) && f->default_is_string) {
     // Resolve the enum's default from a string to an integer.
     upb_byteregion *bytes = upb_value_getbyteregion(f->defaultval);
     assert(bytes);  // Points to either a real default or the empty string.
@@ -347,6 +349,7 @@ static bool upb_fielddef_resolve(upb_fielddef *f, upb_def *def, upb_status *s) {
       }
       upb_value_setint32(&f->defaultval, val);
     }
+    f->default_is_string = false;
     upb_byteregion_free(bytes);
   }
   return true;
@@ -387,11 +390,13 @@ void upb_fielddef_setdefault(upb_fielddef *f, upb_value value) {
 
 void upb_fielddef_setdefaultstr(upb_fielddef *f, const void *str, size_t len) {
   assert(upb_isstring(f) || f->type == UPB_TYPE(ENUM));
-  upb_byteregion *bytes = upb_value_getbyteregion(f->defaultval);
-  assert(bytes);
-  upb_byteregion_free(bytes);
+  if (f->default_is_string) {
+    upb_byteregion *bytes = upb_value_getbyteregion(f->defaultval);
+    assert(bytes);
+    upb_byteregion_free(bytes);
+  }
   upb_value_setbyteregion(&f->defaultval, upb_byteregion_newl(str, len));
-  f->default_is_symbolic = true;
+  f->default_is_string = true;
 }
 
 void upb_fielddef_setdefaultcstr(upb_fielddef *f, const char *str) {
diff --git a/upb/def.h b/upb/def.h
index 67e478b9b7..462655a5fe 100644
--- a/upb/def.h
+++ b/upb/def.h
@@ -109,7 +109,7 @@ typedef struct _upb_fielddef {
   uint8_t label;         // Use UPB_LABEL() constants.
   int16_t hasbit;
   uint16_t offset;
-  bool default_is_symbolic;
+  bool default_is_string;
   bool active;
   int32_t number;
   char *name;
@@ -161,7 +161,7 @@ INLINE upb_value upb_fielddef_default(const upb_fielddef *f) {
 // returns true, the default returned from upb_fielddef_default() is a string,
 // otherwise it is an integer.
 INLINE bool upb_fielddef_default_is_symbolic(const upb_fielddef *f) {
-  return f->default_is_symbolic;
+  return f->default_is_string;
 }
 
 // The enum or submessage def for this field, if any.  Only meaningful for