diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 4d674abcf..0e5827e85 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -313,9 +313,15 @@ int mbedtls_x509write_crt_der( mbedtls_x509write_cert *ctx, unsigned char *buf,
     c = tmp_buf + sizeof( tmp_buf );
 
     /* Signature algorithm needed in TBS, and later for actual signature */
-    pk_alg = mbedtls_pk_get_type( ctx->issuer_key );
-    if( pk_alg == MBEDTLS_PK_ECKEY )
+
+    /* There's no direct way of extracting a signature algorithm
+     * (represented as an element of mbedtls_pk_type_t) from a PK instance. */
+    if( mbedtls_pk_can_do( ctx->issuer_key, MBEDTLS_PK_RSA ) )
+        pk_alg = MBEDTLS_PK_RSA;
+    else if( mbedtls_pk_can_do( ctx->issuer_key, MBEDTLS_PK_ECDSA ) )
         pk_alg = MBEDTLS_PK_ECDSA;
+    else
+        pk_alg = MBEDTLS_PK_NONE;
 
     if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
                                         &sig_oid, &sig_oid_len ) ) != 0 )
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 8fd856b2a..fd22c2890 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -194,14 +194,21 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, s
      */
     mbedtls_md( mbedtls_md_info_from_type( ctx->md_alg ), c, len, hash );
 
-    pk_alg = mbedtls_pk_get_type( ctx->key );
-    if( pk_alg == MBEDTLS_PK_ECKEY )
-        pk_alg = MBEDTLS_PK_ECDSA;
-
     if( ( ret = mbedtls_pk_sign( ctx->key, ctx->md_alg, hash, 0, sig, &sig_len,
-                         f_rng, p_rng ) ) != 0 ||
-        ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
-                                        &sig_oid, &sig_oid_len ) ) != 0 )
+                                 f_rng, p_rng ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    if( mbedtls_pk_can_do( ctx->key, MBEDTLS_PK_RSA ) )
+        pk_alg = MBEDTLS_PK_RSA;
+    else if( mbedtls_pk_can_do( ctx->key, MBEDTLS_PK_ECDSA ) )
+        pk_alg = MBEDTLS_PK_ECDSA;
+    else
+        pk_alg = MBEDTLS_PK_NONE;
+
+    if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
+                                                &sig_oid, &sig_oid_len ) ) != 0 )
     {
         return( ret );
     }