diff --git a/src/gsm/apn.c b/src/gsm/apn.c
index 8d4b2bf..f8303db 100644
--- a/src/gsm/apn.c
+++ b/src/gsm/apn.c
@@ -53,6 +53,9 @@
 	char *str = out_str;
 	size_t rest_chars = apn_enc_len;
 
+	if (!apn_enc)
+		return NULL;
+
 	while (rest_chars > 0 && apn_enc[0]) {
 		size_t label_size = apn_enc[0];
 		if (label_size + 1 > rest_chars)
diff --git a/tests/gprs/gprs_test.c b/tests/gprs/gprs_test.c
index be80e5c..6f16fb2 100644
--- a/tests/gprs/gprs_test.c
+++ b/tests/gprs/gprs_test.c
@@ -91,6 +91,13 @@
 	}
 
 	{
+		/* NULL input */
+		uint8_t input[] = { 0x1, 65 };
+		char *output = "LOL";
+		OSMO_ASSERT(osmo_apn_to_str(output, NULL, ARRAY_SIZE(input) - 1) == NULL);
+	}
+
+	{
 		uint8_t input[] = { 0x3, 65, 66, 67, 0x2, 90, 122 };
 		const char *output = "ABC.Zz";
 		char tmp[strlen(output) + 1];