diff --git a/library/Osmocom_Types.ttcn b/library/Osmocom_Types.ttcn
index a736e95a5..ab5c098d0 100644
--- a/library/Osmocom_Types.ttcn
+++ b/library/Osmocom_Types.ttcn
@@ -83,6 +83,7 @@ function bool2bit_tmpl(template boolean inp) return template BIT1 {
 	} else if (istemplatekind(inp, "*")) {
 		return *;
 	} else if (istemplatekind(inp, "?")) {
+		return ?;
 	} else {
 		if (valueof(inp)) {
 			return '1'B;