diff --git a/lib/dns/tests/tsig_test.c b/lib/dns/tests/tsig_test.c
index 5e0f39bfbd..eaba3d2861 100644
--- a/lib/dns/tests/tsig_test.c
+++ b/lib/dns/tests/tsig_test.c
@@ -75,7 +75,7 @@ add_tsig(dst_context_t *tsigctx, dns_tsigkey_t *key, isc_buffer_t *target) {
 	isc_stdtime_t now;
 	unsigned char tsigbuf[1024];
 	unsigned int count;
-	unsigned int sigsize;
+	unsigned int sigsize = 0;
 	isc_boolean_t invalidate_ctx = ISC_FALSE;
 
 	memset(&tsig, 0, sizeof(tsig));
@@ -110,6 +110,7 @@ add_tsig(dst_context_t *tsigctx, dns_tsigkey_t *key, isc_buffer_t *target) {
 	isc_buffer_init(&sigbuf, tsig.signature, sigsize);
 	CHECK(dst_context_sign(tsigctx, &sigbuf));
 	tsig.siglen = isc_buffer_usedlength(&sigbuf);
+	ATF_CHECK_EQ(sigsize, tsig.siglen);
 
 	CHECK(isc_buffer_allocate(mctx, &dynbuf, 512));
 	CHECK(dns_rdata_fromstruct(&rdata, dns_rdataclass_any,