diff --git a/tests/src/psa_exercise_key.c b/tests/src/psa_exercise_key.c
index 4f08835..3705bc5 100644
--- a/tests/src/psa_exercise_key.c
+++ b/tests/src/psa_exercise_key.c
@@ -623,7 +623,7 @@
                                        psa_algorithm_t alg )
 {
     psa_key_derivation_operation_t operation = PSA_KEY_DERIVATION_OPERATION_INIT;
-    unsigned char input[1];
+    unsigned char input[1] = { 0 };
     unsigned char output[1];
     int ok = 0;
     psa_algorithm_t kdf_alg = PSA_ALG_KEY_AGREEMENT_GET_KDF( alg );