diff --git a/crypto/rsa/rsa_impl.c b/crypto/rsa/rsa_impl.c
index 8dd59dc4..d98ca0da 100644
--- a/crypto/rsa/rsa_impl.c
+++ b/crypto/rsa/rsa_impl.c
@@ -636,6 +636,11 @@ static int mod_exp(BIGNUM *r0, const BIGNUM *I, RSA *rsa, BN_CTX *ctx) {
   r1 = BN_CTX_get(ctx);
   m1 = BN_CTX_get(ctx);
   vrfy = BN_CTX_get(ctx);
+  if (r1 == NULL ||
+      m1 == NULL ||
+      vrfy == NULL) {
+    goto err;
+  }
 
   {
     BIGNUM local_p, local_q;