diff --git a/http/data_source.go b/http/data_source.go
index f91b48a5..de8c9bba 100644
--- a/http/data_source.go
+++ b/http/data_source.go
@@ -124,7 +124,7 @@ func isContentTypeAllowed(contentType string) bool {
 	for _, r := range allowedContentTypes {
 		if r.MatchString(parsedType) {
 			charset := strings.ToLower(params["charset"])
-			return charset == "" || charset == "utf-8"
+			return charset == "" || charset == "utf-8" || charset == "us-ascii"
 		}
 	}