diff --git a/library/pk_internal.h b/library/pk_internal.h
index a51482e..7c4f285 100644
--- a/library/pk_internal.h
+++ b/library/pk_internal.h
@@ -46,7 +46,7 @@
         case MBEDTLS_PK_ECKEY:
         case MBEDTLS_PK_ECKEY_DH:
         case MBEDTLS_PK_ECDSA:
-            return (mbedtls_ecp_keypair *) (pk).MBEDTLS_PRIVATE(pk_ctx);
+            return (const mbedtls_ecp_keypair *) (pk).MBEDTLS_PRIVATE(pk_ctx);
         default:
             return NULL;
     }