diff --git a/libclamav/jsparse/js-norm.c b/libclamav/jsparse/js-norm.c
index ad3814206..486dea2c8 100644
--- a/libclamav/jsparse/js-norm.c
+++ b/libclamav/jsparse/js-norm.c
@@ -718,6 +718,8 @@ static void handle_df(const yystype *tokens, size_t start, struct decode_result
 	if(!str)
 		return;
 	len = strlen(str);
+	if(!len)
+		return;
 	clast = str[len-1] - '0';
 
 	str[len-1] = '\0';