1 files changed, 12 insertions, 1 deletions

diff --git a/src/composite.c b/src/composite.c
index 53e6930b5f2..05365cfb65e 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1746,7 +1746,18 @@ should be ignored.  */)
       CHECK_STRING (string);
       validate_subarray (string, from, to, SCHARS (string), &frompos, &topos);
       if (! STRING_MULTIBYTE (string))
-        error ("Attempt to shape unibyte text");
+        {
+          ptrdiff_t i;
+
+          for (i = SBYTES (string) - 1; i >= 0; i--)
+            if (!ASCII_CHAR_P (SREF (string, i)))
+              error ("Attempt to shape unibyte text");
+          /* STRING is a pure-ASCII string, so we can convert it (or,
+             rather, its copy) to multibyte and use that thereafter.  */
+          Lisp_Object string_copy = Fconcat (1, &string);
+          STRING_SET_MULTIBYTE (string_copy);
+          string = string_copy;
+        }
       frombyte = string_char_to_byte (string, frompos);
     }