diff --git a/lib/hcl.h b/lib/hcl.h
index 18ac7af..385c22f 100644
--- a/lib/hcl.h
+++ b/lib/hcl.h
@@ -1263,10 +1263,16 @@ struct hcl_io_cciarg_t
 	 */
 	void* handle;
 
+	/**
+	 * [OUT] set this to non-zero in HCL_IO_OPEN if the handler fills the buffer with bytes.
+	 *       the caller issues HCL_IO_READ_BYTES if it's set to non-zero, expecting bytes.
+	 *       otherwise it issues HCL_IO_READ expecting characters.
+	 */
+	int is_bytes;
+
 	/**
 	 * [OUT] place data here for #HCL_IO_READ or #HCL_IO_READ_BYTES
 	 */
-	int is_bytes; /* set this to non-zero if the handler fills the buffer with bytes */
 	union
 	{
 		hcl_ooch_t c[HCL_CCI_BUF_LEN];
@@ -1317,6 +1323,11 @@ struct hcl_io_udiarg_t
 	 */
 	void* handle;
 
+	/**
+	 * [OUT] indicates if HCL_IO_READ_BYTES is implemented
+	 */
+	int is_bytes;
+
 	/**
 	 * [OUT] place data in c for #HCL_IO_READ and in d for #HCL_IO_READ_BYTES
 	 */
@@ -1335,7 +1346,6 @@ struct hcl_io_udiarg_t
 	 * Internal use only. Don't touch these.
 	 */
 	hcl_oow_t pos;
-	int is_byte;
 };
 
 typedef struct hcl_io_udoarg_t hcl_io_udoarg_t;
diff --git a/lib/prim.c b/lib/prim.c
index e6d56d0..1cb5119 100644
--- a/lib/prim.c
+++ b/lib/prim.c
@@ -224,14 +224,7 @@ static int get_udi_char (hcl_t* hcl, hcl_ooch_t* ch)
 		hcl->io.udi_arg.xlen = 0;
 		if (hcl->io.udi_rdr(hcl, HCL_IO_READ, &hcl->io.udi_arg) <= -1) return -1;
 		if (hcl->io.udi_arg.xlen <= 0) return 0; /* EOF */
-		hcl->io.udi_arg.is_byte = 0;
-	}
-
-
-	if (hcl->io.udi_arg.is_byte)
-	{
-		/* TODO: set error */
-		return -1;
+		hcl->io.udi_arg.is_bytes = 0;
 	}
 
 	*ch = hcl->io.udi_arg.buf.c[hcl->io.udi_arg.pos++];
@@ -240,19 +233,21 @@ static int get_udi_char (hcl_t* hcl, hcl_ooch_t* ch)
 
 static int get_udi_byte (hcl_t* hcl, hcl_uint8_t* bt)
 {
+#if defined(HCL_OOCH_IS_UCH)
+	if (!hcl->io.udi_arg.is_bytes)
+	{
+		hcl_seterrbfmt (hcl, HCL_EPERM, "prohibited byte-oriented input");
+		return -1;
+	}
+#endif
+
 	if (hcl->io.udi_arg.pos >= hcl->io.udi_arg.xlen)
 	{
 		hcl->io.udi_arg.pos = 0;
 		hcl->io.udi_arg.xlen = 0;
 		if (hcl->io.udi_rdr(hcl, HCL_IO_READ_BYTES, &hcl->io.udi_arg) <= -1) return -1;
 		if (hcl->io.udi_arg.xlen <= 0) return 0; /* EOF */
-		hcl->io.udi_arg.is_byte = 1;
-	}
-
-	if (!hcl->io.udi_arg.is_byte)
-	{
-		/* TODO: set error */
-		return -1;
+		hcl->io.udi_arg.is_bytes = 1;
 	}
 
 	*bt = hcl->io.udi_arg.buf.b[hcl->io.udi_arg.pos++];