diff --git a/library/pk.c b/library/pk.c
index ec3741b..097777f 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -1126,6 +1126,12 @@
         return mbedtls_pk_verify(ctx, md_alg, hash, hash_len, sig, sig_len);
     }
 
+    /* Ensure the PK context is of the right type otherwise mbedtls_pk_rsa()
+     * below would return a NULL pointer. */
+    if (mbedtls_pk_get_type(ctx) != MBEDTLS_PK_RSA) {
+        return MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
+    }
+
 #if defined(MBEDTLS_RSA_C) && defined(MBEDTLS_PKCS1_V21)
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     const mbedtls_pk_rsassa_pss_options *pss_opts;