diff --git a/main/pbx.c b/main/pbx.c
index 3cb9affeb..e625a873a 100644
--- a/main/pbx.c
+++ b/main/pbx.c
@@ -3732,6 +3732,9 @@ void ast_str_substitute_variables_full(struct ast_str **buf, ssize_t maxlen, str
 	ast_str_reset(*buf);
 	whereweare = tmp = templ;
 	while (!ast_strlen_zero(whereweare)) {
+		/* reset our buffer */
+		ast_str_reset(substr3);
+
 		/* Assume we're copying the whole remaining string */
 		pos = strlen(whereweare);
 		nextvar = NULL;
diff --git a/tests/test_substitution.c b/tests/test_substitution.c
index 98e1ed191..366b9371c 100644
--- a/tests/test_substitution.c
+++ b/tests/test_substitution.c
@@ -263,6 +263,7 @@ AST_TEST_DEFINE(test_substitution)
 	TEST(test_expected_result(test, c, "${LISTFILTER(list1,&,cd)}", "ab&ef"));
 	TEST(test_expected_result(test, c, "${SHELL(echo -n 123)},${SHELL(echo -n 456)}", "123,456"));
 	TEST(test_expected_result(test, c, "${foo},${CDR(answer)},${SHELL(echo -n 456)}", "123,,456"));
+	TEST(test_expected_result(test, c, "${foo},${this_does_not_exist},${THIS_DOES_NOT_EXIST(either)}", "123,,"));
 #undef TEST
 
 	/* For testing dialplan functions */