diff --git a/main/mio.c b/main/mio.c
index 075b1621d1..1e924cdb4c 100644
--- a/main/mio.c
+++ b/main/mio.c
@@ -361,12 +361,12 @@ MIO *mio_new_memory (unsigned char *data,
  * @size: the length of the data copied from @base to new mio
  *
  * Creates a new #MIO object by copying data from existing #MIO (@base).
- * The range for copying are given with @start and @size.
+ * The range for copying is given with @start and @size.
  * Copying data at the range from @start to the end of @base is
- * done if 0 is given as @size.
+ * done if -1 is given as @size.
  *
- * If @size(!= 0) is larger than the length from @start to the end of
- * @base, %NULL is return.
+ * If @size is larger than the length from @start to the end of
+ * @base, %NULL is returned.
  *
  * The function doesn't move the file position of @base.
  *
@@ -374,7 +374,7 @@ MIO *mio_new_memory (unsigned char *data,
  *
  */
 
-MIO *mio_new_mio (MIO *base, long start, size_t size)
+MIO *mio_new_mio (MIO *base, long start, long size)
 {
 	unsigned char *data;
 	long original_pos;
@@ -383,7 +383,7 @@ MIO *mio_new_mio (MIO *base, long start, size_t size)
 
 	original_pos = mio_tell (base);
 
-	if (size == 0)
+	if (size == -1)
 	{
 		long end;
 
diff --git a/main/mio.h b/main/mio.h
index e78ad8962f..b9186f788b 100644
--- a/main/mio.h
+++ b/main/mio.h
@@ -123,7 +123,7 @@ MIO *mio_new_memory (unsigned char *data,
 					 MIOReallocFunc realloc_func,
 					 MIODestroyNotify free_func);
 
-MIO *mio_new_mio    (MIO *base, long start, size_t size);
+MIO *mio_new_mio    (MIO *base, long start, long size);
 MIO *mio_ref        (MIO *mio);
 
 int mio_free (MIO *mio);
diff --git a/main/parse.c b/main/parse.c
index 66306dd6fc..2ac40454fe 100644
--- a/main/parse.c
+++ b/main/parse.c
@@ -854,7 +854,7 @@ struct getLangCtx {
 		    (mio_memory_get_data((_glc_)->input, NULL) == NULL)) \
 		{							\
 			MIO *tmp_ = (_glc_)->input;			\
-			(_glc_)->input = mio_new_mio (tmp_, 0, 0);	\
+			(_glc_)->input = mio_new_mio (tmp_, 0, -1);	\
 			mio_free (tmp_);				\
 			if (!(_glc_)->input) {				\
 				(_glc_)->err = true;			\