diff --git a/src/sas.c b/src/sas.c
index 75e7724..7117fed 100644
--- a/src/sas.c
+++ b/src/sas.c
@@ -141,6 +141,10 @@ size_t olm_sas_calculate_mac(
         sas->last_error = OLM_OUTPUT_BUFFER_TOO_SMALL;
         return (size_t)-1;
     }
+    if (!sas->their_key_set) {
+        sas->last_error = OLM_SAS_THEIR_KEY_NOT_SET;
+        return (size_t)-1;
+    }
     uint8_t key[32];
     _olm_crypto_hkdf_sha256(
         sas->secret, sizeof(sas->secret),
@@ -164,6 +168,10 @@ size_t olm_sas_calculate_mac_long_kdf(
         sas->last_error = OLM_OUTPUT_BUFFER_TOO_SMALL;
         return (size_t)-1;
     }
+    if (!sas->their_key_set) {
+        sas->last_error = OLM_SAS_THEIR_KEY_NOT_SET;
+        return (size_t)-1;
+    }
     uint8_t key[256];
     _olm_crypto_hkdf_sha256(
         sas->secret, sizeof(sas->secret),