diff --git a/src/composite.c b/src/composite.c
index 4d69702171f..fb0c51821a5 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1879,11 +1879,7 @@ Otherwise (for terminal display), FONT-OBJECT must be a terminal ID, a
 	  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;
+	  /* STRING is a pure-ASCII string, so we can treat it as multibyte.  */
 	}
       frombyte = string_char_to_byte (string, frompos);
     }
diff --git a/src/lisp.h b/src/lisp.h
index 7be2e5d38dc..9f2e7785d2f 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1637,12 +1637,10 @@ #define STRING_SET_UNIBYTE(STR)				\
 
 /* Mark STR as a multibyte string.  Assure that STR contains only
    ASCII characters in advance.  */
-#define STRING_SET_MULTIBYTE(STR)			\
-  do {							\
-    if (XSTRING (STR)->u.s.size == 0)			\
-      (STR) = empty_multibyte_string;			\
-    else						\
-      XSTRING (STR)->u.s.size_byte = XSTRING (STR)->u.s.size; \
+#define STRING_SET_MULTIBYTE(STR)			    \
+  do {							    \
+    eassert (XSTRING (STR)->u.s.size > 0);		    \
+    XSTRING (STR)->u.s.size_byte = XSTRING (STR)->u.s.size; \
   } while (false)
 
 /* Convenience functions for dealing with Lisp strings.  */