diff --git a/string.c b/string.c
index 515a340adc..e5db1feef6 100644
--- a/string.c
+++ b/string.c
@@ -1500,7 +1500,10 @@ rb_str_modify_expand(VALUE str, long expand)
 	    str_make_independent_expand(str, expand);
 	}
     }
-    ENC_CODERANGE_CLEAR(str);
+    if (ENC_CODERANGE(str) == ENC_CODERANGE_BROKEN) {
+	/* Force re-scan later */
+	ENC_CODERANGE_CLEAR(str);
+    }
 }
 
 /* As rb_str_modify(), but don't clear coderange */