diff --git a/crypto/evp/internal.h b/crypto/evp/internal.h
index 72b46f8..8b755d0 100644
--- a/crypto/evp/internal.h
+++ b/crypto/evp/internal.h
@@ -89,7 +89,7 @@
   int pkey_base_id;
   unsigned long pkey_flags;
 
-  char *pem_str;
+  const char *pem_str;
 
   int (*pub_decode)(EVP_PKEY *pk, X509_PUBKEY *pub);
   int (*pub_encode)(X509_PUBKEY *pub, const EVP_PKEY *pk);