diff --git a/src/character.h b/src/character.h
index 3642a54044..75672f8d54 100644
--- a/src/character.h
+++ b/src/character.h
@@ -122,6 +122,10 @@ #define MAKE_CHAR_MULTIBYTE(c) \
 /* This is the maximum byte length of multibyte form.  */
 #define MAX_MULTIBYTE_LENGTH 5
 
+/* Nonzero iff C is an ASCII byte.  */
+#define ASCII_BYTE_P(c) UNSIGNED_CMP (c, <, 0x80)
+
+
 /* Nonzero iff X is a character.  */
 #define CHARACTERP(x) (FIXNATP (x) && XFIXNAT (x) <= MAX_CHAR)
 
diff --git a/src/composite.c b/src/composite.c
index 364d5c9316..121a554754 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1744,20 +1744,45 @@ Otherwise (for terminal display), FONT-OBJECT must be a terminal ID, a
 
   if (NILP (string))
     {
-      if (NILP (BVAR (current_buffer, enable_multibyte_characters)))
-	error ("Attempt to shape unibyte text");
-      validate_region (&from, &to);
+     validate_region (&from, &to);
       frompos = XFIXNAT (from);
       topos = XFIXNAT (to);
-      frombyte = CHAR_TO_BYTE (frompos);
+      if (!NILP (BVAR (current_buffer, enable_multibyte_characters)))
+	      frombyte = CHAR_TO_BYTE (frompos);
+      else
+      {
+	      ptrdiff_t pos;
+
+	      /* fill_gstring_header below uses
+		 FETCH_CHAR_ADVANCE_NO_CHECK that assumes the current
+		 buffer is multibyte, but it is safe as long as it only
+		 fetches ASCII chars.  */
+	      for (pos = frompos; pos < topos; pos++)
+		      if (!ASCII_BYTE_P (*(BYTE_POS_ADDR (pos))))
+			      error ("Attempt to shape non-ASCII part of unibyte text");
+	      frombyte = frompos;
+      }
+
     }
   else
     {
       CHECK_STRING (string);
       validate_subarray (string, from, to, SCHARS (string), &frompos, &topos);
-      if (! STRING_MULTIBYTE (string))
-	error ("Attempt to shape unibyte text");
-      frombyte = string_char_to_byte (string, frompos);
+      if (STRING_MULTIBYTE (string))
+	      frombyte = string_char_to_byte (string, frompos);
+      else
+      {
+	      ptrdiff_t pos;
+
+	      /* fill_gstring_header below uses
+		 FETCH_STRING_CHAR_ADVANCE_NO_CHECK that assumes the
+		 string is multibyte, but it is safe as long as it only
+		 fetches ASCII chars.  */
+	      for (pos = frompos; pos < topos; pos++)
+		      if (!ASCII_BYTE_P (SREF (string, pos)))
+			      error ("Attempt to shape non-ASCII part of unibyte text");
+	      frombyte = string_char_to_byte (string, frompos);
+      }
     }
 
   header = fill_gstring_header (frompos, frombyte,